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Title: Inheritance of Characteristics in Domestic Fowl
Author: Davenport, Charles Benedict
Language: English
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INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL.
BY
CHARLES B. DAVENPORT,
DIRECTOR OF THE STATION FOR EXPERIMENTAL EVOLUTION, CARNEGIE
INSTITUTION OF WASHINGTON.
[Illustration]
WASHINGTON, D. C.
PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON. 1909
CARNEGIE INSTITUTION OF WASHINGTON PUBLICATION NO. 121.
PAPERS OF THE STATION FOR EXPERIMENTAL EVOLUTION, NO. 14.
PRESS OF J. B. LIPPINCOTT COMPANY PHILADELPHIA
TABLE OF CONTENTS.
PAGE
INTRODUCTION 3
CHAPTER I. THE SPLIT OR Y COMB 5
A. Interpretation of the Y Comb 5
B. Variability of the Y Comb and Inheritance of the
Variations 12
CHAPTER II. POLYDACTYLISM 17
A. Types of Polydactylism 17
B. Results of Hybridization 18
CHAPTER III. SYNDACTYLISM 29
A. Statement of Problem 29
B. Results of Hybridization 32
CHAPTER IV. RUMPLESSNESS 37
CHAPTER V. WINGLESSNESS 42
CHAPTER VI. BOOTING 43
A. Types of Booting 43
B. Normal Variability 43
C. Results of Hybridization 46
CHAPTER VII. NOSTRIL-FORM 59
CHAPTER VIII. CREST 67
CHAPTER IX. COMB-LOP 69
CHAPTER X. PLUMAGE COLOR 71
A. The Gametic Composition of the Various Races 71
1. White 71
2. Black 72
3. Buff 72
B. Evidence 72
1. Silkie × Minorca (or Spanish) 72
2. Silkie × White Leghorn 75
3. Silkie × Buff Cochin 76
4. White Leghorn × Black Minorca 77
5. White Leghorn × Buff Cochin 77
6. Black Cochin × Buff Cochin 78
CHAPTER XI. INHERITANCE OF BLUE COLOR, SPANGLING, AND BARRING 79
A. Blue Color 79
B. Spangling 80
C. Barring 81
1. White Cochin × Tosa 81
2. White Leghorn Bantam × Dark Brahma 82
3. White Leghorn Bantam × Black Cochin 82
CHAPTER XII. GENERAL DISCUSSION 85
A. Relation of Heredity and Ontogeny 85
B. Dominance and Recessiveness 88
C. Potency 92
D. Reversion and the Factor Hypothesis 93
E. The Limits of Selection 94
1. Increasing the Red in the Dark Brahma × Minorca
Cross 94
2. Production of a Buff Race by Selection 95
F. Non-inheritable Characters 96
G. The Rôle of Hybridization in Evolution 97
LITERATURE CITED 99
===============================================
INHERITANCE OF CHARACTERISTICS
IN DOMESTIC FOWL.
BY
CHARLES B. DAVENPORT.
===============================================
INTRODUCTION.
A series of studies is here presented bearing on the question of
dominance and its varying potency. Of these studies, that on the Y
comb presents a case where relative dominance varies from perfection
to entire absence, and through all intermediate grades, the average
condition being a 70 per cent dominance of the median element. When
dominance is relatively weak or of only intermediate grade the
second generation of hybrids contains extracted pure dominants in
the expected proportions of 1:2:1; but as the potency of dominance
increases in the parents the proportion of offspring with the
dominant (single) comb increases from 25 per cent to 50 per cent.
This leads to the conclusion that, on the one hand, dominance varies
quantitatively and, on the other, that the degree of dominance is
inheritable.
The studies on polydactylism reveal a similar variation of potency in
dominance and show, in Houdans at least, an inheritance of potency
(table 11), and moreover they suggest a criticism of Castle's
conclusion of inheritance of the degree of polydactylism.
Syndactylism illustrates another step in the series of decreasing
potency of the dominant. On not one of the F1 generation was the
dominant (syndactyl) condition observed; and when these hybrids were
mated together the dominant character appeared not in 75 per cent but
in from 10 per cent to 0 per cent of the offspring. The question may
well be asked: What is then the criterion of dominance? The reply
is elaborated to the effect that, since dominance is due to the
presence of a character and recessiveness to its absence, dominance
may fail to develop, but recessiveness never can do so. Consequently
two extracted recessives mated _inter se_ can not throw the dominant
condition; but two imperfect dominants, even though indistinguishable
from recessives, will throw dominants. On the other hand, owing to
the very fact that the dominant condition often fails of development,
two extracted "pure" dominants will, probably always, throw some
apparent "recessives." Now, two syndactyls have not been found that
fail (in large families) to throw normals, but extracted normals
have been found which, bred _inter se_, throw only normals; hence,
"normal-toe" is recessive. In this character, then, dominance almost
always fails to show itself in the heterozygote and often fails in
pure dominants.
The series of diminishing potency has now brought us to a point
where we can interpret a case of great difficulty, namely, a case of
rumplessness. Here a dominant condition was originally mistaken for
a recessive condition, because it never fully showed itself in F1
and F2. Nevertheless, in related individuals, the condition is fully
dominant. We thus get the notion that a factor that normally tends to
the development of a character may, although present, fail to develop
the character. Dominance is lacking through _impotence_.
The last term of the series is seen in the wingless cock which left
no wingless offspring in the F1 and F2 generations. In comparison
with the results gained with the rumpless cock, winglessness in this
strain is probably dominant but impotent.
When a character, instead of being simply present or absent, is
capable of infinite gradations, inheritance seems often to be
blending and without segregation. Two cases of this sort--booting
and nostril-height--are examined, and by the aid of the principle
of imperfect dominance the apparent blending is shown to follow
the principle of segregation. Booting is controlled by a dominant
inhibiting factor that varies greatly in potency, and nostril-height
is controlled by an inhibiting factor that stops the over-growth of
the nasal flap which produces the narrow nostril.
The extracted dominants show great variability in their progeny, but
the extracted recessives show practically none. This is because a
positive character may fail to develop; but an absent character can
not develop even a little way. The difference in variability of the
offspring of two extracted recessives and two extracted dominants
is the best criterion by which they may be distinguished, or by
which the presence (as opposed to the absence) of a factor may be
determined.
The crest of fowl receives especial attention as an example of a
character previously regarded as simple but now known to comprise two
and probably more factors--a factor for erectness, one for growth,
and probably one or more that determine the restriction or extension
of the crested area.
The direction of lop of the single comb is an interesting example
of a character that seems to be undetermined by heredity. In this
it agrees with numerous right and left handed characters. It is not
improbable that the character is determined by a _complex_ of causes,
so that many independent factors are involved.
A series of studies is presented on the inheritance of plumage color.
It is shown that each type of bird has a gametic formula that is
constant for the type and which can be used with success to predict
the outcome of particular combinations. New combinations of color
and "reversions" receive an easy explanation by the use of these
factors. The cases of blue, spangled, and barred fowl are shown also
to contain mottling or spangling factors.
CHAPTER I.
THE SPLIT OR Y COMB.
A. INTERPRETATION OF THE Y COMB.
When a bird with a single comb, which may be conveniently symbolized
as I, is crossed with a bird with a "V" comb such as is seen in
the Polish race, and may be symbolized as oo, the product is a
split or Y comb. This Y comb is a _new form_. As we do not expect
new forms to appear in hybridization, the question arises, How is
this Y comb to be interpreted? Three interpretations seem possible.
According to one, the antagonistic characters (allelomorphs) are I
comb and oo comb, and in the product neither is recessive, but both
dominant. The result is a case of particulate inheritance--the single
comb being inherited anteriorly and the oo comb posteriorly. On
this interpretation the result is not at all Mendelian.
According to the second interpretation the hereditary units are
not what appear on the surface, but each type of comb contains two
factors, of which (in each case) one is positive and the other
negative. In the case of the I comb the factors are presence of
median element and absence of lateral or paired element; and in the
case of the oo comb the factors are absence of median element and
presence of lateral element. On this hypothesis the two positive
factors are dominant and the two negative factors are recessive.
The third hypothesis is intermediate between the others. According
to it the germ-cells of the single-combed bird contain a median unit
character which is absent in the germ-cells of the Polish or Houdan
fowl. This hypothesis supposes further that the absence of the median
element is accompanied by a fluctuating quantity of lateral cere, the
so-called V comb.
The split comb is obtained whenever the oo comb is crossed with a
type containing the median element. Thus, the offspring of a oo
comb and a pea comb is a split pea comb, and the offspring of a
oo comb and a rose comb is a split rose. The three hypotheses may
consequently be tested in three cases where a split comb is produced.
TABLE 1.
+-----------------------+-----+-----+------------+
| | I | Y | No median. |
+-----------------------+-----+-----+------------+
| I × I | 100 | 0 | 0 |
| I × Y | 50 | 50 | 0 |
| I × no median | 0 | 100 | 0 |
| Y × no median | 0 | 50 | 50 |
| No median × no median | 0 | 0 | 100 |
+-----------------------+-----+-----+------------+
The first and third hypotheses will give the same statistical result,
namely, the products of two Y-combed individuals of F1 used as
parents, will exhibit the following proportions: median element, 25
per cent; split comb, 50 per cent; and no median element, 25 per
cent. These proportions will show themselves, whatever the generation
to which the Y-combed parents belong, whether both are of
generation F1, or F2, or F3, or one parent of one generation and the
other of another. Other combinations of parental characters should
give the proportions in the progeny shown in table 1.
On the second hypothesis, on the other hand, the proportions of
the different kinds occurring in the progeny will vary with the
generation of the parents. This hypothesis assumes the existence in
each germ-cell of the original parent of two comb allelomorphs, _M_
and _l_ in single-combed birds and _m_ and _L_ in the Polish fowl,
the capital letter standing for the presence of a character (Median
element or Lateral element) and the small letter for the absence of
that character. Consequently, after mating, the zygote of F1 contains
all 4 factors, _MmLl_, and the soma has a Y comb; but in the
germ-cells, which contain each only 2 unlike factors, these factors
occur in the following 4 combinations, so that there are now 4 kinds
of germ-cells instead of the 2 with which we started. These are _ML_,
_Ml_, _mL_, and _ml_. Furthermore, since in promiscuous mating of
birds these germ-cells unite in pairs in a wholly random fashion, 16
combinations are possible, giving 16 F2 zygotes (not all different)
as shown in table 2.
TABLE 2.
+-------+---------------+---------+
| Type. | Zygotic | Soma. |
| | constitution. | |
+-------+---------------+---------+
| _a_ | M2L2[A] | Y |
+-------+---------------+---------+
| _b_ | M2Ll | Y |
+-------+---------------+---------+
| _c_ | MmL2 | Y |
+-------+---------------+---------+
| _d_ | MmLl | Y |
+-------+---------------+---------+
| _e_ | M2Ll | Y |
+-------+---------------+---------+
| _f_ | M2l2 | I |
+-------+---------------+---------+
| _g_ | MmLl | Y |
+-------+---------------+---------+
| _h_ | Mml2 | I |
+-------+---------------+---------+
| _i_ | mLML | Y |
+-------+---------------+---------+
| _k_ | mLMl | Y |
+-------+---------------+---------+
| _l_ | m2L2 | oo |
+-------+---------------+---------+
| _m_ | m2Ll | oo |
+-------+---------------+---------+
| _n_ | mlML | Y |
+-------+---------------+---------+
| _o_ | mlMl | I |
+-------+---------------+---------+
| _p_ | m2Ll | oo |
+-------+---------------+---------+
| _q_ | m2l2 | Absent |
+-------+---------------+---------+
[A] This convenient form of zygotic formulæ, using a
subscript 2 instead of doubling the letter, is proposed
by Prof. W. E. Castle.
It is a consequence of this second hypothesis that, in F2, of every
16 young 9 should have the Y comb; 3 the I comb; 3 the oo comb,
and 1 no comb at all. It follows further that the progeny of two F2
parents will differ in different families. Thus if a Y-combed bird
of type _a_ be mated with a bird of any type, _all_ of the progeny
will have the Y comb.
From Y-combed parents of various types taken at random 4 kinds of
families will arise having the following percentage distribution of
the different types of comb:
1. Y comb, 100 per cent.
2. Y comb, 75 per cent; I comb, 25 per cent.
3. Y comb, 75 per cent; oo comb, 25 per cent.
4. Y comb, 56.25 per cent; I comb, 18.75 per cent; oo comb,
18.75 per cent; absent, 6.25 per cent.
Again, mating two extracted I combs of F2 should yield, in F3, two
types of families in equal frequency as follows:
1. I comb, 100 per cent.
2. I comb, 75 per cent; no comb, 25 per cent.
Again, mating two extracted oo combs of F2 should yield, in F3, two
types of families in equal frequency, as follows:
1. oo comb, 100 per cent.
2. oo comb, 75 per cent; no comb, 25 per cent.
Single comb × Y comb should give families of the types:
1. Y comb, 100 per cent.
2. Y comb, 50 per cent; I comb, 50 per cent.
3. Y comb, 50 per cent; oo comb, 50 per cent.
4. Y comb, 25 per cent; I comb, 25 per cent; oo comb, 25
per cent; absent, 25 per cent.
Mating oo comb and Y comb should give the family types:
1. Y comb, 100 per cent.
2. Y comb, 50 per cent; oo comb, 50 per cent.
3. Y comb, 50 per cent; I comb, 50 per cent.
4. Y comb, 25 per cent; oo comb, 25 per cent; I comb, 25
per cent; no comb, 25 per cent.
Finally, I comb and oo comb should give the following types of
families:
1. Y comb, 100 per cent.
2. I comb, 100 per cent.
3. Y comb, 50 per cent; oo comb, 50 per cent.
4. I comb, 50 per cent; no comb, 50 per cent.
Now, what do the facts say as to the relative value of these three
hypotheses? Abundant statistics give a clear answer. In the first
place, the progeny of two Y-combed F1 parents is found to show the
following distribution of comb types: Y comb 471, or 47.3 per cent;
I comb 289, or 29.0 per cent; oo comb 226, or 22.7 per cent; and
no comb 10, or 1 per cent. The presence of no comb in F2 speaks for
the second hypothesis, but instead of the 6.25 per cent combless
expected on that hypothesis only 1 per cent appears. There is no
close accord with expectation on the second hypothesis.
Coming now to the F3 progeny of two Y-combed parents, we get the
distribution of families shown in table 3.
TABLE 3.
+--------+-------------------------+---------------------------+
| Pen No.| Parents. | Comb in offspring. |
| |-------------------------+------+-----+-----+--------|
| | ♀ (F2) | ♂ (F2) | I | Y | oo | Absent.|
|--------+-------------+-----------+------+-----+-----+--------+
| 707 | { 366 | 1378 | 18 | 16 | 9 | .... |
| | { 522 | 1378 | 1 | 1 | 0 | .... |
| | | | | | | |
| | { 2250 | 2247 | 9 | 5 | 4 | 1 |
| 763 | { 2700 | 2247 | 3 | 5 | 3 | 1 |
| | { 3799 | 2247 | 5 | 4 | 3 | .... |
| | | | | | | |
| 769 | { 1305 | 911 | 7 | 4 | 6 | .... |
| | { 2254 | 911 | 15 | 15 | 7 | .... |
| |------+-----+-----+--------+
| Totals (142) | 58 | 50 | 32 | 2 |
| Proportions (per cent) | 40.8 | 35.2| 22.5| 1.4 |
| | | | ⎿⎵⎵⎵⏌ |
| | | | 23.9 |
+---------+-------------+----------+------+-----+--------------+
An examination of these families shows not one composed exclusively
of Y-combed individuals nor those (of significant size) containing
Y-combed and I-combed or oo-combed individuals exclusively,
much less in the precise proportion of 3:1, yet such should
be the commonest families if the second hypothesis were true.
Notwithstanding the marked deviation--to be discussed later--from
the expected proportions of I, 25 per cent; Y, 50 per cent;
oo, 25 per cent, the result accords better with the first or third
hypothesis. Since on either of these hypotheses the same proportions
of the various types of comb are to be expected in the progeny of
Y-combed parents of whatever generation, it is worth recording that
from such parents belonging to all generations except the first the
results given in table 4 were obtained, and it will be noticed that
these results approach expectation on the first or third hypothesis.
TABLE 4.
+-----------+-------+-------+-------+---------+--------+
| | I | Y | oo | Absent. | Total. |
+-----------+-------+-------+-------+---------+--------+
|Frequency | 235 | 291 | 144 | 12 | 682 |
| | | | | | |
|Percentage | 34.5 | 42.7 | 21.1 | 1.8 | .... |
+-----------+-------+-------+-------+---------+--------+
The progeny of two extracted single-combed parents of the F2
generation give in 3 families the following totals: Of 95 F3
offspring, 94 have single combs; one was recorded from an unhatched
chick as having a _slightly_ split comb, but this was probably a
single comb with a slight side-spur, a form that is associated with
purely I-combed germ-cells. This result is in perfect accord with
the second and third hypotheses, but is irreconcilable with the first
hypothesis.
The progeny of two extracted oo-combed parents is given in table 5.
TABLE 5.
+-----+-----------------+----------------------+
| | Parents. | Comb in offspring. |
| Pen +--------+--------+----+----+----+-------+
| No. | ♀ | ♂ | I | Y | oo |Absent.|
| | (F2). | (F2). | | | | |
+-----+--------+--------+----+----+----+-------+
| 729 |{2255 | 936 | .. |[A]4| 36| .. |
| |{2269 | 936 | .. | .. | 29| .. |
| | | | | | | |
| |{ 369 | 1390 | 1 | .. | 3| .. |
| 756 |{1067 | 1390 | .. | .. | 8| 1 |
| |{1113 | 1390 | .. | .. | 13| 4 |
| | | | | | | .. |
| |{2011 | 444 | .. | .. | 10| .. |
| |{2011 | 2621 | .. | .. | 9| .. |
| |{2333 | 444 | .. |[A]5| 11| .. |
| |{2333 | 2621 | .. |[A]1| 2| .. |
| 762 |{2618 | 444 | .. | .. | 2| .. |
| |{2618 | 2621 | .. | .. | 5| .. |
| |{3776 | 444 | .. | .. | 2| .. |
| |{3776 | 2621 | .. | 1 | 14| .. |
| | | | | | | |
| |{2016 | 4731 | .. | .. | 10| .. |
| 820 |{2255 | 4731 | .. | .. | 16| .. |
| |{5143 | 4731 | .. | .. | 45| .. |
| |{6479 | 4731 | .. | .. | 31| .. |
| | | | | | | |
| |{[B]2618| 5119 |[B]1| .. | 23| .. |
| |{3776 | 5119 | .. | .. | 28| .. |
| 832 |{4404 | 5119 | .. | .. | 9| .. |
| |{4732 | 5119 | .. | .. | 3| .. |
| |{5803 | 5119 | .. | .. | 21| 2 |
| |{6481 | 5119 | .. | .. | 11| .. |
| | | | | | | |
| 834 | 2324 | 5090 | .. | .. | 26| .. |
| +----+----+---+--------+
| Total | 2 | 11 | 367| 7 |
+--------------+--------+----+----+------------+
[A] Median element recorded as "small" in these offspring.
[B] A median element visible in the mother, No. 2618.
The distribution of offspring in the 24 families of table 5 is
in fair accord with any of the three hypotheses, but seems to
favor the second, for that hypothesis calls for families with
combless children, whereas such are not to be expected on the first
hypothesis. Moreover, agreement with the second hypothesis is fairly
close, for that calls for 3 families with combless children and
there were actually 3 such families showing a total of 1.8 per cent
combless, where expectation is 2.8 per cent. What is opposed to
any hypothesis is the appearance of some Y-combed offspring; and
to account for this the hypothesis is suggested that the germ-cells
of some parents with oo comb contain traces of the I-comb
determiner. The word "traces" is used because the median element in
these Y-combed offspring is practically always very small. It is
fair, consequently, to conclude that oo × oo gives oo-combed,
and occasionally combless, offspring. This conclusion is further
supported by the statistics derived from extracted oo comb of _all_
generations bred _inter se_, which give: Y 11, oo 427, and no
comb 8, where the 11 Y-combed birds are those just referred to as
progeny of F2 parents. The non-median comb, consequently, probably
contains only non-median germ-cells.
TABLE 6.
+---+-------------------------------------------------+--------------+
| | Parents. | Offspring. |
| |--------+-----+----------+------+-----+----------+----+----+----+
|Pen| ♀ |Form | Degree | ♂ |Form | Degree | | | |
|No.| (F2). | of | of | (F2).| of | of | I | Y | oo |
| | |comb.|splitting.| |comb.|splitting.| | | |
|---|--------|-----|----------|------|-----|----------|----|----|----|
| | | | _P. ct._ | | | _P. ct._ | | | |
| | { 427 | Y | 5 | 439 | I | 0 | 5 | 1 | .. |
|628| { 722 | Y | 20 | 439 | I | 0 | 1 | 5 | .. |
| | { 725 | Y | 10 | 439 | I | 0 | 5 | 3 | .. |
| | | | | | | | | | |
|629| 427 | I | 0 | 491 | Y | 50 | 9 | 6 | .. |
| | | | | | | | | | |
|765| 1790 | I | 0 | 1794 | Y | 90 | 17 | 25 | .. |
| | | | | | | | | | |
| | {3846 | I | 0 | 6652 | Y | 90 | 8 | 5 | .. |
|802| {5025 | I | 0 | 6652 | Y | 90 | 14 | 11 | 2 |
| | {5087 | I | 0 | 6652 | Y | 90 | 13 | 17 | 2 |
| | | | | | | | | | |
|812| {4254 | I | 0 | 4118 | Y | 90 | 15 | 13 | .. |
| | {5540 | I | 0 | 4118 | Y | 90 | 8 | 9 | .. |
| |----|----|----|
| Totals (189) | 95 | 95 | 4 |
| Percentages |49.0|49.0| 2.0|
+-----------------------------------------------------+----+----+----+
The mating of extracted I comb and Y comb, both of the second (or
later) hybrid generation, gives the following distribution of types
in the offspring (table 6): Y comb 95 (49 per cent); I comb 95
(49 per cent); oo comb 4 (2 per cent). In detail the results given
in table 6 accord badly with the second hypothesis, which demands
some families with 100 per cent Y comb.
The mating of extracted oo comb×Y comb, where both parents are of
the second hybrid generation, gave the distribution of comb types in
the 6 families that are recorded in table 7.
TABLE 7.
+---+-----------------+-------------------------+
| | Parents. | Offspring. |
|Pen|-----------------+-----+-----+-----+-------|
|No.| ♀ | ♂ | I | Y | oo |Absent.|
| | (F2). | (F2). | | | | |
+------------+--------|-----+-----|-----|-------|
|634| { 298 | 444 | 0 | 15 | 18 | .... |
| | { 366 | 444 | 5 | 23 | 15 | .... |
| | | | | | | |
|729| { 913 | 936 | 2 | 28 | 37 | .... |
| | { 935 | 936 | ....| 13 | 39 | .... |
| | | | | | | |
|756| { 1043 | 1390 | ....| 13 | 11 | 1 |
| | { 1048 | 1390 | ....| 0 | 5 | .... |
| |-----+-----+-----+-------+
| Totals (214) | 7 | 92 | 115 | 1 |
+---------------------+-----+-----+-----+-------+
The single comb recorded in the case of 7 birds is doubtless merely
the limiting condition of a Y comb in which the median element is
developed to its fullest extent. All but 2 of the 7 were recorded
from early embryos when an incipient bifurcation would be more
difficult to detect. This explanation applies generally, and accounts
for the usual excess of I comb when compared with Y comb, as
for instance in table 3, page 7. Returning to table 7, it is,
consequently, probable that only the Y-combed and non-median-combed
offspring are produced and that they are in the proportion of 99 to
115 or of 46 per cent to 54 per cent. If we add together all records
of a oo×Y cross, disregarding the generation of the parents, we
get a total I comb 5,[1] Y comb 177, oo comb 172, and absent 3,
or 182 (51 per cent) with the median element and 175 (49 per cent)
without. Thus the oo×Y cross gives the 1:1 proportion called for
on the first and third hypotheses and not at all the variety required
by the second hypothesis.
[1] Excluding 6 doubtful because from too young embryos
and not observed by myself.
TABLE 8.
+---+----------------------+---------------+------------------+
| | Mother. | Father. |Comb in offspring.|
|Pen+---------+-----+------+--------+------+---+---+----+-----+
|No.| | |P. ct.| | | | | | |
| | No. |Comb.|split.| No. |Comb. | I | Y | oo | Abs.|
+---+---------+-----+------+--------+------+---+---+----+-----+
|704| { 65 F1| Y | 50 | 1420 F2|Absent|.. | 10| 6 | 8 |
| | {1061 F2| Y | 50 | 1420 F2| Do. |.. | 4| .. | 1 |
| | | | | | | | | | |
|819| { 57 F1| Y | 50 | 1420 F2| Do. |.. | 8| 6 | 5 |
| | { 65 F1| Y | 60 | 1420 F2| Do. |.. | 1| .. | 1 |
| +---+---+----+-----+
| Total | 0 | 23| 12 | 15 |
+------------------------------------------+---+---+----+-----+
Finally, we must consider the result of mating a bird without
papillæ (No. 1420, pen 704) with a median-combed hen (480). When
this typical single-combed hen was used the 49 progeny were all of
the Y type.[2] This proves that the combless type behaves only as
an extreme of the non-median type.
[2] One is reported as having a I comb; probably the limiting
condition, again.
When Y-combed hens were used with the combless cock the offspring
had Y comb and non-median-comb in nearly equal numbers, 23:27
(table 8), but the latter included an unusually large proportion
of combless fowl (15 in 27). When a combless hen (No. 4257) was
used, 9 of the offspring had oo comb and 2 no comb; not a greater
proportion of combless birds than in the no-comb×Y-combed cross.
All of these facts indicate that "comblessness" is not entire absence
of the comb factors, but a minimum case of the oo or paired comb.
This result is opposed to the second hypothesis.
The statistics of all matings between I, Y, and no comb on the
one side and no comb on the other thus speak unanimously for the
conclusion that in these matings we are not dealing with 2 pairs
of allelomorphs, but with a single comb and its absence (third
hypothesis) or with a case of particulate inheritance (first
hypothesis). Moreover, it must be said that the split comb is
obtained also when the Polish-Houdan comb is crossed with a pea
comb or a rose comb; and the pea and rose combs can not be said to
have "lateral comb absent," as required by the second hypothesis.
Consequently the second hypothesis is definitely excluded.
It now remains to decide between the two remaining hypotheses. First
of all, it may be said that the perfection with which I and oo
combs can be extracted from Y-combed birds indicates that we are
here dealing with a case of Mendelian inheritance and, in so far,
favors the third hypothesis. To accord with the theory of particulate
inheritance, of which the first hypothesis is a special case, the two
united characters should transmit the mosaic purely; but this they do
not do. Hence the third hypothesis is to be preferred to the first.
Comblessness is a necessary consequence of the second hypothesis and
is inexplicable on the first hypothesis. On the third hypothesis
it may be accounted for as follows: Absence of single comb is
allelomorphic to its presence. The lateral comb is a character common
to fowl either with or without the median comb, but it is ordinarily
repressed in the birds with single comb and gains a large size when
the median element is absent. It is a very variable element. At one
extreme it forms the cup comb; at the other there is an absence of
any trace of comb. My own records show all grades between these
extremes, including minute papillæ on both sides of the head or on
one side only, low paired ridges, the butterfly comb, and cup comb
shorter than normal. This variability of the lateral element is
comparable to the fluctuation in size of the single comb itself,
as illustrated by the Single-comb Minorca on the one hand and the
Cochin on the other. It is comparable, also, to the fluctuation in
the paired part of the Y comb, which we shall consider in the next
section, and to the variability of the oo comb as met with in the
pens of fanciers.
The foregoing considerations do not, at first sight, account for the
Y comb as seen in F1. Yet they provide us with all the data for an
explanation. Median comb of the Minorca dominates over no median of
the Polish, and so in F1 we have the median element represented. But,
on the well-known principle of imperfection of dominance in F1, the
median comb is usually incomplete and, probably for some ontogenetic
reason, incomplete only behind. The incompleteness behind permits the
development there of the elsewhere repressed lateral comb, and we
therefore have the Y comb--evidence at the same time of a repressed
lateral-comb Anlage in the single-combed birds and of imperfection of
dominance of the single comb in the first hybrid generation.
B. VARIABILITY OF THE Y COMB AND INHERITANCE OF THE VARIATIONS.
[Illustration: FIG A.--The frequency of the different forms of Y
comb, each form being based on the percentage of the median element
of the Y comb to the entire length of comb.]
As already stated, the proportions of the median and the lateral
elements in the Y comb are very variable; the median element may,
indeed, constitute anywhere from 100 per cent to 0 per cent of the
entire comb. Even full brothers and sisters show this variability.
Thus the offspring of No. 13 ♀ Single-comb Minorca and No.
3 ♂ Polish have the median element of the Y comb ranging
from 0 per cent to 70 per cent of the whole comb. Notwithstanding
this variability of the median element in any family there is a
difference in the average and the range of variability in families
where different races are employed. Thus the offspring of two Polish
× Minorca crosses show an _average_ of 46 per cent of the median
element in the comb; the Houdan × Minorca cross gives combs with 60
per cent of the median element; and in the combs of the offspring of
two Houdan × White Leghorn crosses there is, on the average, 71 per
cent of the median element. The Houdan × Dark Brahma (pea comb) gives
combs with an average of 87 per cent median element and the Polish
× Rose-comb Minorca cross gives 89 per cent median. The rose-combed
hens used in this last cross were heterozygous, having single comb
recessive; consequently they produced also chicks with typical Y
combs. Such had, on the average, only 59 per cent of the median
element and were thus in striking contrast with the slightly split
rose combs. In the case of the partially split rose combs the median
element ranged from 60 per cent to 100 per cent of the whole length
of the comb; but in the split single combs the range is from 0 to
100 per cent. Thus, in the two cases, the proportion of the median
element and the range of its variability differ greatly.
Also, in generations subsequent to the first, the Y comb exhibits
this same variability. We have already seen that the progeny of the
Y-combed offspring of any generation may be compared with those
of any other, and so we may mass together the progeny of all hybrid
generations so long as they are derived from the same ancestral pure
races.
In inquiring into the meaning of this variability we must first
construct the polygon of frequency of the various grades of median
element. This is plotted in fig. A, which is a composite whose
elements are, however, quite like the total curve. There is one
empirical mode at 70 per cent and another at 0 per cent. The smaller
mode at 50 per cent is, I suspect, due to the tendency to estimate
in round numbers, and may be, in this discussion, neglected. From
this polygon we draw the conclusions, first, that the median element
in the Y comb tends to dominate strongly over the absence of this
element, as 7:3, and, second, that dominance is rarely complete. Yet
there is an important number of cases, even in F1, where the median
element is almost or completely repressed (down to 10 to 0 per cent
of the whole) and the comb consists of two high and long lateral
elements--the "cup comb" of Darwin. There are, then, in the offspring
of a median-combed and a non-median-combed parent, two types with few
intergrades--the type of slightly incomplete dominance of the median
element and the type of very incomplete dominance.
We have now to consider how these two types of comb and their
fluctuations behave in heredity. When two parents having each combs
of the 70 per cent or 80 per cent median type are mated, their
offspring belong to the three categories of I, Y, and "no-median"
comb, but the relative frequency of these three categories is not
close to the ideal of 25 per cent, 50 per cent, and 25 per cent,
respectively. For there is actually in 336 offspring a marked
excess of the I comb, 36 per cent, 44 per cent, and 20 per cent,
respectively, resulting. When, on the other hand, two parents having
each combs of the 10 per cent and 0 per cent types are mated their
offspring are of the same three categories and the proportions
actually found in 241 offspring (28 per cent, 47 per cent, 25 per
cent) closely approximate the ideal. It is clear, then, that even the
cup comb, without visible median element, has such an element in its
germ-cells and is totally different in its hereditary behavior from
the Polish comb, in which the median element is absent, not only from
the soma, but also from the germ-cells.
We have seen in the last paragraph that the Y comb with only 10
per cent to 0 per cent median element has germ-cells bearing median
comb as truly as the Y comb containing 70 per cent to 80 per
cent median element, but we have also seen that in the latter case
there is an excess of single-combed progeny. We have now to inquire
whether, in general, there is a close relation between the proportion
of median element in the comb of the parents and the percentage of
single-combed offspring. These relations are brought out in the lower
half of table 9.
TABLE 9.--_Frequency of the different proportions of single
element in the combs of offspring of parents having the average
proportion of median element given in the column at the left._
+-------------+--------------------------------------------------------+
| | Y combs. |
| +--------------------------------------------------------+
| | Offspring. |
| +----+----+----+----+----+----+----+----+----+----+------+
| | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 |Total.|
+-------------+----+----+----+----+----+----+----+----+----+----+------+
| { 0 | 21 | 5 | 4 | 3 | 4 | 6 | 5 | 10 | 8 | 1 | 67 |
| {10 | 21 | 5 | 3 | 0 | 3 | 9 | 2 | 4 | 2 | 0 | 49 |
| {20 | 5 | 4 | 2 | 1 | 0 | 4 | 2 | 12 | 0 | 1 | 31 |
| {30 | 8 | 17 | 8 | 10 | 9 | 22 | 12 | 30 | 8 | 3 | 127 |
| Parents {40 | 9 | 7 | 4 | 2 | 7 | 39 | 18 | 46 | 26 | 5 | 163 |
| {50 | 7 | 5 | 2 | 1 | 5 | 32 | 13 | 48 | 35 | 11 | 159 |
| {60 | 10 | 7 | 2 | 2 | 2 | 19 | 14 | 47 | 51 | 15 | 169 |
| {70 | 9 | 2 | 4 | 0 | 1 | 6 | 7 | 28 | 41 | 11 | 109 |
| {80 | .. | .. | 1 | 1 | 1 | 1 | 6 | 12 | 11 | 6 | 39 |
| {90 | .. | 2 | 1 | 0 | 0 | 3 | 0 | 3 | 8 | 9 | 26 |
| +----+----+----+----+----|----+----+----+----+----+------+
| Total | 90 | 54 | 31 | 20 | 32 |141 | 79 |240 |190 | 62 | 939 |
+======================================================================+
| | All types of combs in offspring. |
| +----------+---------------------------------------------+
| |Number | I | Y | Non-median. |
| |of +------+--------+------+--------+-----+-------+
| |offspring.| No. | P. ct. | No. | P. ct. | No. | P. ct.|
|-------------+----------+------+--------+------+--------+-----+-------+
| { 0 | 146 | 42 | 20 | 67 | 46 | 37 | 25 |
| {10 | 99 | 25 | 25 | 49 | 50 | 25 | 25 |
| {20 | 73 | 22 | 30 | 31 | 43 | 20 | 27 |
| {30 | 249 | 61 | 25 | 127 | 51 | 61 | 24 |
| Parents {40 | 309 | 73 | 24 | 163 | 53 | 73 | 23 |
| {50 | 329 | 93 | 28 | 159 | 48 | 77 | 23 |
| {60 | 368 | 120 | 33 | 169 | 46 | 79 | 21 |
| {70 | 232 | 80 | 35 | 109 | 47 | 43 | 18 |
| {80 | 104 | 42 | 40 | 39 | 38 | 23 | 22 |
| {90 | 75 | 38 | 51 | 26 | 34 | 11 | 15 |
| +----------+------+--------+------+--------+-----+-------+
| Total | 1984 | 596 | 30.0 | 939 | 47.3 | 449 | 22.7|
+-------------+----------+------+--------+------+--------+-----+-------+
The proportion of single-combed offspring in the total filial
population is 30.0 per cent, a departure of such magnitude from the
expected 25 per cent as to arrest our attention. Further inspection
of table 9 shows that the excess of single-combed offspring is found
only in the lower half of the series. When the percentage of median
element in the parents is under 50 the proportions of I, Y, and
no-median combs are as 25.5 per cent, 49.8 per cent, 24.7 per cent,
or close to expectation; but when the percentage is 50 or over the
proportions are, on the average, 33.6 per cent, 45.2 per cent, and
21.2 per cent, a wide departure from expectation, 1108 individuals
being involved. An examination of table 9 shows, moreover, that
the proportion of offspring with single comb rises steadily as the
proportion of the median element in the parentage increases from 50
per cent. The meaning of this fact is at present obscure, but the
suspicion is awakened that, while the "cup comb" and the more deeply
split combs are typical heterozygotes the slightly split combs are
a complex of 2 or more units, one of which is "single comb." But
that this is not the explanation follows for two reasons: first, that
even in the F1 generation slightly split combs are obtained, and,
second, that the offspring of the cup combs are much more variable
than those of slightly split combs (70 to 90 per cent median). What
is strikingly true is that, from 50 per cent up, as the proportion
of the median element in the parents increases the percentage of
single-combed offspring rises.
The matter may be looked at in another light. Median comb is dominant
over its absence. Typically, we should expect F1 to show a single
comb; the Y comb that we actually get is a heterozygous condition
due to the failure of the median comb to dominate completely.
Typically we should expect F2 to reveal 75 per cent single combs,
of which 1 in 3 is homozygous and 2 in 3 are heterozygous. Owing
to the failure of single comb always to dominate completely in the
heterozygotes, we expect to find some of the 75 per cent with the
Y comb. When in the parents dominance has been very incomplete in
the heterozygote (as is the case in the 0 per cent to 40 per cent
median-combed parents) we find it so in the offspring also and all
heterozygotes show a Y comb of some type. But when in the parents
dominance has been strong in the heterozygote (50 per cent to 90
per cent) it is so in the offspring also and only a part of the
heterozygotes show the Y comb; the others show the single comb and
thus swell the numbers of the single-combed type. The only objection
to this explanation is found in the reduction in the percentages
of the no-median type. Thus, adding together the homozygous and
heterozygous median-combed offspring and comparing with the
non-median-combed, we find these ratios:
Parental per cent 0-40 50 60 70 80 90
Ratio 75.3:24.7 76:23 79:21 82:18 78:22 85:15
There is a great deviation from 25 per cent in the "non-median"
offspring of the 90 per cent parents, but in this particular case
the total number of offspring is not large, and the deviation has a
greater chance of being accidental. Altogether this explanation of
the varying per cents of single comb on the ground of inheritance of
varying potency in dominance seems best to fit the facts of the case.
From the foregoing facts and considerations we may conclude that the
Y comb represents imperfect dominance of median over no-median
comb; that there is a fluctuation in the potency of the dominance, so
that the proportion of the median element varies from 0 to over 90
per cent; that the more potent the dominance of median element is in
any parents the more complete will be the dominance in the offspring
and the smaller will be the percentage of imperfectly dominant, or
Y-combed, offspring. _Dominance varies quantitatively and the
degree of dominance is inheritable._
The index of heredity may be readily obtained in the familiar
biometric fashion from table 9. This I have calculated and found
to be 0.301± 0.002. This agrees with Pearson's theoretical
coefficient of correlation between offspring and parent. The index
is larger than it would otherwise be because it is measured with
an _average_ of the parents and these parents assortatively mated.
But this instance is, in any case, an interesting example of strong
inheritance of a quantitative variation.
What, it may be asked, is the relation of these facts to the general
principle that inheritance is through the gametes? Why, when a gamete
with the median element unites with a gamete without that element,
does the zygote develop a soma that in some cases shows a nine-tenths
median and sometimes a one-tenth median element? We have seen that
the Y comb is a heterozygous form due to imperfection of dominance
of the median element; but why this variation in the perfection of
the median element? This is probably a piece of the question, why
any dominance at all. We find, in general, that the determiner of a
well-developed organ dominates in the zygote over the determiner of a
slightly developed condition of that organ or its obsolete condition.
It is as though there were in the zygote an interaction between the
strong and the weak form of the determiner, and the strong won; but
sometimes the victory is imperfect. In the specific case of comb the
interaction between median and no-median leads to a modification,
weakening, or imperfection of the median element, and this weakening
varies in degree. Sometimes the weakening is inappreciable--when the
comb is essentially single; sometimes it is great, and the result is
a comb in which the median element is reduced to one-half; sometimes,
finally, the determiner of median comb is so completely weakened
by its dilution with "no-median" as not to be able to develop,
and we have the cup comb with only a trace of the median element.
Nevertheless, such a cup comb is heterozygous and produces both
single-combed and Polish-combed germ-cells. Thus the variation in the
extent of the median comb seems to point to variations in relative
potency of the median comb over its absence.
CHAPTER II.
POLYDACTYLISM.
The possession of extra toes is a character that crops out again and
again among the higher, typically 5-toed vertebrates. Many cases have
been cited in works on human and mammalian teratology (_cf._ Bateson,
1904, and Schwalbe, 1906), and it is recognized that this abnormality
is very strongly inherited in man. Bateson and Saunders, and Punnett
(1902 and 1905), Hurst (1905), and Barfurth (1908), as well as myself
in my earlier report, have demonstrated the inheritableness of the
character in poultry. Bateson and Punnett (1905, p. 114) say: "The
normal foot, though commonly recessive, may sometimes _dominate_ over
the extra-toe character, and this heterozygote may give equality
when bred with recessives, just as if it were an ordinary DR."
Altogether, the inheritance of extra-toe diverges so far from typical
Mendelian results as to deserve further study.
A. TYPES OF POLYDACTYLISM.
There are two main types of polydactylism: that in which the inner
toe (I) of the normal foot is replaced by 2 simple toes, and
that in which it is replaced by two toes, of which the mediad is
simple and the laterad is divided distally. The former type is
characteristic of the Houdans; the latter is usually associated with
the Silkies. Both conditions are, however, found in both races. The
simplest condition is seen in many Houdans of my strain. It consists
of 2 equal, medium-sized toes (I' and I") lying close together
and parallel to or slightly convex towards each other. This condition
indicates that the 2 toes, together, are to be regarded as the
equivalent of the normal single toe occupying the same position. The
2 toes are, I conjecture, derived from the single toe by splitting.
The first series of changes consists of the increase in length of the
lateral element (I") and a corresponding decrease of the median
element (I'). In the last term of the series there are only 4 toes
on the foot, but the inner toe is not like the normal inner toe of
poultry, but is a much elongated I".
In the Silkie, also, the series begins with 2 small, closely-applied
toes (I' and I"). But when there are only 2 toes the lateral one
is usually much the larger. Typically this lateral toe is, as stated,
split, so that the nail is double, and the degree of splitting is
variable, in extreme cases involving half or more than half of the
toe. A second series of changes consists of the gradual reduction of
toe I' (often concomitantly with an increase in I") which may
end in its entire disappearance and thus reduce the number of toes to
5, but these are not equivalent to the 5 toes of the Houdans, since
the extra Houdan toes are I', I", and those of the reduced Silkie
are I"_a_ and I"_b_. Finally, in Silkies, the inner toe (I')
may split (more or less completely), and thus the 7-toed condition
arises. Moreover, in Houdans I have on one or two occasions found
the lateral element (I") bifid distally, resembling perfectly the
typical condition found in the Silkies.
A simple nomenclature is suggested for these various types of
extra-toes. The simple double-toed condition, as found commonly
in Houdans, may be called the _duplex_ type (D). The loss of
I' gives the _reduced duplex_ (D'). The case of split I", as
commonly seen in the Silkie, is the _triplex_ type (T); with the
loss of I' this becomes the _reduced triplex_ (T', not duplex!).
The 7-toed condition of Silkies may be called the _quadruplex type_
(Q); the combination split I' and single I" gives the _reduced
quadruplex_ (Q').[3]
[3] _E. g._, Pen 813, 935 ♀, embryo from egg of May 13.
The reduction that leads to the loss of I' consists of a loss
of phalanges, as Bateson (1904) has already pointed out. It seems
probable that the reduction affects first the proximal phalanges,
since the distal nail-bearing phalanx is the last to disappear.
B. RESULTS OF HYBRIDIZATION.
First let us consider the result of mating extra-toed individuals
belonging to "pure" extra-toed races. A typical Houdan cock (D
type), of the well-known Petersen strain, was mated with 3 hens bred
by me, but derived, several generations before, from the same strain.
With the first hen he got 29 chicks, all with the extra-toe except
one (3.3 per cent) that had 4 toes on both feet and two that had 4
toes on one foot and 5 on the other, _i. e._, one foot simplex and
one duplex. With the second he got 12 chicks, of which one had 4-5
(D) toes. The third, in 26 young, gave one with 4 toes on each
foot. Thus, in 67 chicks altogether there were 2, or 3 per cent,
with the normal number of toes on both feet (4-4). Unfortunately
these birds did not survive, so it is not known whether they would
have thrown as large a proportion of extra-toed offspring as
5-toed Houdans. Bateson's Dorkings gave about 4 per cent of 4-toed
offspring. Of the 83 offspring of 6-toed Silkies, 3, or 3.6 per
cent, had 4 toes on each foot. Even in pure-bred polydactyl races,
consequently, the character "extra-toe" does not uniformly appear in
the offspring.
Let us consider next what happens when a polydactyl individual is
crossed with a normal individual. Table 10 gives the results of
all matings of this sort and its most obvious result is that the
polydactyl condition reappears in every family, but not, as in
typically Mendelian cases, in _all_ of the offspring; at least this
is true of the Houdan crosses. In the Silkie crosses the 6 offspring
given as having the single thumb may possibly have been of the type
D', as that type was not in mind at the time of making the record
and was not always distinguished from type S. It is also clear
that the offspring of Silkie crosses are more apt to be polydactyl
than those of Houdan crosses. For 27 per cent of the latter are
non-polydactyl, while, taking the table as it stands, at most only
about 4 per cent and (as just stated) probably none of the Silkie
offspring were of the typical single-thumbed type. Also the average
degree of polydactylism is much greater in the Silkie than in the
Houdan crosses. This excess is in part due to the different method
of counting toes in the Silkie and the Houdan hybrids; for whereas
in the latter the visible toes are counted as equivalent units,
in the former in the case of each reduced type one unit more is
assigned than appears. The actual number of toes occurring in the
Silkie hybrids was also calculated, and it was found that this still
averaged higher than that of the Houdans (9.45 as opposed to 9.26).
TABLE 10.--_Frequency of the various types of toes in the first
hybrid generation between a normal and an extra-toed parent._
+-------------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES. |
+-----+--------------------------------------+---------------------------------+------------------------------+
| Pen | Mother. | Father. | Offspring. |
| No. +-----------+----------------+---------+---------------------------------+---------------------+--------+
| | No. | Race involved. | No. of | No. | Race involved. | No. of | Types of toes. |Average.|
| | | | toes. | | | toes. | 4-4 | 4-5 | 5-5 | |
+-----+-----------+----------------+---------+-------+----------------+--------+-------+------+------+--------+
| | {8 or 11 | Houdan. | 5-5 | } | | | { 0 | 1 | 8 | 9.9 |
| 504 | { 8 | Do. | 5-5 | } 13 | Wh. Leghorn. | 4-4 | { 1 | 3 | 8 | 9.6 |
| | { 11 | Do. | 5-5 | } | | | { 2 | 2 | 7 | 9.5 |
| | | | | | | | | | | |
| 525 | 8 or 11 | Do. | 5-5 | 27 | Minorca. | 4-4 | 8 | 3 | 13 | 9.2 |
| | | | | | | | | | | |
| 727 | { "Y" | Dk. Brahma. | 4-4 | } 831 | Houdan. | 5-5 | { 3 | 2 | 5 | 9.2 |
| | { 121 | Do. | 4-4 | } | | | { 13 | 9 | 18 | 9.1 |
| | | | | | | | | | | |
| 504 | 10-12 | Wh. Leghorn. | 4-4 | 9 | Do. | 5-5 | 3 | 2 | 0 | 8.4 |
| +-------+------+------+--------+
| Total (110) | 30 | 21 | 59 | 9.26 |
| Percentages | 27.3| 19.1| 53.6| |
+------------------------------------------------------------------------------+-------+------+------+--------+
| B. SILKIE CROSSES. |
+----+--------------------+-------------------------+---------------------------------------------------------+
| Pen| Mother. | Father. | Offspring. |
| No.+-----+-------+------+-----+------------+------+-------------------------------------------------+-------+
| | No. | Race. |No. of| No. | Race. |No. of| Types of toes.[A] | Aver- |
| | | | toes.| | | toes+---+----+---+-----+----+---+----+-----+----+-----+ age. |
| | | | | | | |ss.|sd'.|sd.|d'd'.|d'd.|dd.|st'.|d't'.|dt'.|t't'.| |
+----+-----+-------+------+-----+------------+------+---+----+---+-----+----+---+----+-----+----+-----+-------+
| 851| 1002|Cochin.| 4-4 | 7526|Silkie. | 6-6 |.. | .. | 1 | .. | 1 | 2 | .. | .. | 2 | 3 | 10.78 |
| 851| 3410| Do. | 4-4 | 7526| Do. | 6-6 |1? | .. |.. | .. | 2 | 7 | .. | .. | 1 | 3 | 10.43 |
| 815| 131| Do. | 4-4 | 774| Do. | 6-6 |.. | .. |.. | 1 | .. | 8 | .. | 1 | 1 | 1 | 10.33 |
| 851| 2073| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | .. | 7 | 1 | .. | .. | 1 | 10.33 |
| 734| 841| Do. | 4-4 | 774| Do. | 6-6 |.. | .. |.. | .. | .. | 3 | .. | .. | 1 | .. | 10.25 |
| 851| 838| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. | 1 | 1 | .. |11 | .. | .. | .. | 3 | 10.25 |
| 851| 2299| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. | 1?| 1 | .. | 4 | .. | .. | .. | 1 | 10.14 |
| 851| 5567| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | 1 |10 | 1 | .. | 1 | .. | 10.08 |
| 734| 840| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | 1 | .. | 7 | .. | .. | .. | .. | 10.00 |
| 734| 1002| Do. | 4-4 | 774| Do. | 6-6 |.. | .. |.. | .. | 2 | 8 | .. | .. | .. | .. | 10.00 |
| 851| 840| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | .. | 4 | .. | .. | .. | .. | 10.00 |
| 851| 841| Do. | 4-4 | 7526| Do. | 6-6 |.. | .. |.. | .. | 1 | 1 | .. | .. | .. | .. | 10.00 |
| 744| 777|Silkie.|[B]5-6| 1176|Wh. Leghorn.| 4-4 |.. | .. |.. | .. | .. | 6 | .. | .. | .. | .. | 10.00 |
| 744| 496| Do. | 6-6 | 1176| Do. | 4-4 |1? | .. |.. | .. | .. |12 | .. | .. | 1 | .. | 9.93 |
| 851| 6956|Cochin.| 4-4 | 7526|Silkie. | 6-6 |4? | 1 |.. | 2 | .. | 3 | .. | .. | .. | .. | 9.50 |
| +---+----+---+-----+----+---+----+-----+----+-----+-------+
| Total (138) | 6 | 1 | 3 | 6 | 7 |93 | 2 | 1 | 7 | 12 | 10.13 |
+---------------------------------------------------+---+----+---+-----+----+---+----+-----+----+-----+-------+
[A] Of the reduced triplex type (t').
[B] s, means type of single thumb; d, duplex type; d', reduced duplex; t', reduced triplex.
In hybrids of both classes the greatest number of toes occurring on
one foot never exceeds the greatest number possessed by its parents;
indeed, the most polydactyl hybrids of the F1 generation of Silkies
never have as many as 6 toes on one foot. This result is not to be
explained as due to a regression towards the 4-4-toed condition, but
rather as due to the intermediate condition of the heterozygote. For
80 per cent of the hybrids show either the typical or the reduced
D type on one or both feet, although neither parent exhibits these
types.
We have next to consider the results of mating together the F1
hybrids. Table 11 gives the results of all matings of this sort.
TABLE 11.--_Frequency of the various types of toes in the second
hybrid generation between normal and extra-toed races. Lettering
as in table 10._
+------------------------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES (F1 × F1). |
+------+-----+------------------------------+------------------------------+---------------------------------------------+
| | | Mother. | Father. | Offspring. |
| | +-----+------------------+-----+-----+------------------+-----+----------------------------------+----------+
|Serial| Pen | | | | | | | | Average |
| No. | No. | | | No. | | | No. | Types of toes. | num. of |
| | | No. | Races involved. | of | No. | Races involved. | of +------+------+------+------+------+ toes per |
| | | | |toes.| | |toes.| 4-4 | 4-5 | 5-5 | 4-6 | 5-6 | bird. |
+------+-----+-----+------------------+-----+-----+------------------+-----+------+------+------+------+------+----------+
| 1 | 631 | 429 |Houd. × Wh. Legh. | 5-5 | 83 |Wh. Legh. × Houd. | 4-4 | 14[A]| 7 | 28 | 1 | .. | 9.3 |
| 2 | 728 | 174 | Do. | 5-5 | 258 | Do. | 5-5 | 11 | 1 | 20 | .. | .. | 9.3 |
| 3 | 631 | 448 | Do. | 5-5 | 409 | Do. | 4-4 | 13 | 4 | 18 | .. | .. | 9.1 |
| 4 | 637 | 529 |Houd. × Min. | 5-5 | 570 |Houd. × Min. | 4-4 | 4 | .. | 5 | .. | .. | 9.1 |
| 5 | 631 | 430 |Houd. × Wh. Legh. | 4-4 | 83 |Wh. Legh. × Houd. | 4-4 | 20 | 1 | 21 | .. | .. | 9.0 |
| 6 | 631 | 504 |Wh. Legh. × Houd. | 5-5 | 83 | Do. | 4-4 | 27 | 3 | 23 | .. | .. | 8.9 |
| 7 | 631 | 174 |Houd. × Wh. Legh. | 5-5 | 83 | Do. | 4-4 | 14 | 9 | 11 | .. | 1 | 8.9 |
| 8 | 519 | 85 | Do. | 4-5 | 83 | Do. | 4-4 | 9 | 2 | 4 | .. | .. | 8.7 |
| 9 | 637 | 569 |Houd. × Min. | 5-5 | 570 |Houd. × Min. | 4-4 | 14 | 1 | 4 | .. | 1 | 8.7 |
| 10 | 637 | 797 | Do. | 5-5 | 570 | Do. | 4-4 | 2 | .. | 1 | .. | .. | 8.7 |
| 11 | 631 | 86 |Houd. × Wh. Legh. | 4-4 | 83 |Houd. × Wh. Legh. | 4-4 | 11 | 1 | 6 | .. | .. | 8.7 |
| 12 | 637 | 685 |Houd. × Min. | 4-4 | 570 |Houd. × Min. | 4-4 | 5 | 1 | 2 | .. | .. | 8.6 |
| 13 | 631 | 84 |Houd. × Wh. Legh. | 4-4 | 83 |Houd. × Wh. Legh. | 4-4 | 17 | 13 | 4 | .. | .. | 8.6 |
| 14 | 519 | 84 | Do. | 4-4 | 83 | Do. | 4-4 | 7 | 1 | 2 | .. | .. | 8.5 |
| 15 | 519 | 86 |Wh. Legh. × Houd. | 4-4 | 83 |Wh. Legh. × Houd. | 4-4 | 12 | 2 | 2 | .. | .. | 8.4 |
| | | | +------+------+------+------+------+----------+
| | | | Totals (380) |180 | 46 | 151 | 1 | 2 | 8.92 |
| | | | Percentages | 47.4 | 12.1| 39.7| 0.3 | 0.5 | |
+------+-----+-----+-------------------------------------------------------+------+------+------+------+------+----------+
| B. SILKIE CROSSES (F1 × F1). |
+------+---+------------------------+------------------------+-----------------------------------------------------------+
| | | Mother. | Father. | Offspring (F2). |
| | +----+-------------+-----+----+-------------+-----+-----------------------------------------------------------+
|Serial|Pen| | | No. | | | No. | Types of toes. |
| No. |No.| No.| Races. | of | No.| Races. | of +-----------------------------------------------------------+
| | | | |toes.| | |toes.| ss | sd |d'd'| d'd| dd | st |d't'| dt'| dt |t't'| t't| tt |
+------+---+----+-------------+-----+----+-------------+-----+----+----+----+----+----+----+----+----+----+----+----+----+
| 16 |753|2071|Min. × Silk. | 4-4 |2573|Min. × Silk. | 4-5 | 7 | .. | .. | 1 | 19 | .. | 1 | .. | 3 | .. | 1 | .. |
| 17 |753|1966| Do. | 4-4 |2573| Do. | 4-5 | 12 | 2 | .. | .. | 15 | 1 | .. | .. | 2 | .. | .. | 4 |
| 18 |753|2575| Do. | 4-5 |2573| Do. | 4-5 | 18 | .. | 1 | .. | 16 | .. | .. | 1 | .. | .. | .. | 1 |
| 19 |709|3827|Silk. × Span.| 4-4 |1578|Silk. × Span.| 6-5 | 3 | .. | .. | .. | 2 | .. | .. | .. | .. | .. | .. | .. |
| 20 |709|1963| Do. | 4-4 |1578| Do. | 6-5 | 12 | 5 | .. | 1 | 15 | 1 | .. | .. | 1 | .. | .. | 1 |
| 21 |821|7413|Silk. × Coch.| 5-5 |6095|Silk. × Coch.| 5-5 | 1 | .. | .. | 1 | 7 | .. | .. | .. | 2 | .. | .. | .. |
| 22 |821|7423| Do. | 5-5 |6095| Do. | 5-5 | 3 | .. | .. | .. | 7 | .. | .. | .. | .. | 1 | .. | 1 |
| 23 |821|7428| Do. | 5-5 |6095| Do. | 5-5 | 5 | .. | 1 | 4 | 13 | .. | .. | 2 | .. | .. | .. | 1 |
| 24 |821|7408| Do. | 5-5 |6095| Do. | 5-5 | 3 | 1 | .. | .. | 8 | .. | .. | .. | 1 | 1 | .. | .. |
| | | | +----+----+----+----+----+----+----+----+----+----+----+----+
| | | | Total (208) | 64 | 8 | 2 | 7 |102 | 2 | 1 | 3 | 8 | 2 | 1 | 8 |
+------+---+----+--------------------------------------------+----+----+----+----+----+----+----+----+----+----+----+----+
[A]: Includes 1 case of 3-4 toes.
Comparing tables 10 and 11, it is at once clear that in the second
hybrid generation the proportion of extra-toed offspring has
decreased. This accords with expectation, if extra-toe is dominant,
for then only 75 per cent would be of the dominant type in F2, while
100 per cent would be of that type in F1.
Table 12 will enable us to analyze the difference of the proportions
in tables 10 and 11.
TABLE 12.--_Percentages of the various types of toes in F1 and F2
of the polydactyl hybrids compared._
+------+---------------+------------------+------------------+
| | _a_. | _b_. | _c_. |
| |Houdan hybrids.|Silkie hybrids (as|Silkie hybrids (as|
|No. of| | observed). | interpreted).[A]|
|toes. +---------------+------------------+------------------+
| | F1. | F2. | F1. | F2. | F1. | F2. |
+------+-------+-------+-------+----------+-------+----------+
| 4-4 | 27.3 | 47.4 | 9.4 | 31.7 | 4.3 | 30.8 |
| 4-5 | 19.1 | 12.1 | 9.4 | 7.7 | 2.9 | 3.8 |
| 4-6 | .... | .3 | .... | 1.0 | 1.5 | 1.0 |
| 5-5 | 53.6 | 39.7 | 81.2 | 51.4 | 76.8 | 53.4 |
| 5-6 | .... | .5 | .... | 4.3 | 5.8 | 5.8 |
| 6-6 | .... | .... | .... | 3.9 | 8.7 | 5.3 |
+------+-------+-------+-------+----------+-------+----------+
[A] Reduced duplex and triplex toes classified as typical duplex
and triplex.
These tables yield several points of interest. First, although the
proportions of normal and extra toe in table 12, _a_ and _c_, are
not Mendelian, yet the average _increase_, from F1 to F2 in the
proportion of the recessive (4-toed) type is almost exactly what is
called for by Mendel's law. That law calls for an increase of 25
per cent. The actual average increase is 23.3 per cent (20.1 and
26.5 in the two cases). It seems fair to conclude, consequently,
that Mendel's law does hold here, and that the 4-toed individuals
of F1 are heterozygotes with imperfect dominance. The feet of most
of the 4-toed Silkies of this generation belong, indeed, to the
reduced 5-toed type (table 10, B), and the reduced condition is
_prima facie_ evidence of heterozygotism. In F1 Silkies of the
first hybrid generation, 20 per cent of the feet exhibit "reduced"
types of toes, but in F2 only 5 per cent; and this might have been
anticipated, since in F2 heterozygotes are relatively only half as
numerous as in F1. Again, in F2 we see reappearing the high ancestral
toe-numbers (practically lost in the heterozygotes of F1, table 12,
_b_). These I interpret as extracted dominants. 6-toed extracts are
more numerous among the Silkie than the Houdan hybrids, because the
Silkie ancestors were 6-toed and the Houdan ancestors only 5-toed.
However, only a small proportion of the extracted Silkie dominants
have as many toes as the original Silkie ancestors, and this
indicates a permanent regression (through the contaminating influence
of hybridization?) toward the normal condition of toes. It will be
observed that, although 6 toes are not found in the Silkie hybrids
of F1, many of these heterozygotes are of the reduced triplex type.
Classifying them as virtually 6-toed, we find (table 12, _c_) 14.5
per cent of the 6-toed type in the F1 generation.
Among the extracted dominants of F2 are a few showing more toes than
appeared in the ancestors (table 12, _a_; there was also one 7-toed
F2 Silkie hybrid, not recorded in the table). It is this sort of
an advance in F2 that permits the breeder to make a forward step.
Theoretically, the appearance of this more aberrant class is probably
due to the greater numbers of progeny than of ancestors, since the
extracted dominants of F2 are seven times as numerous as their
extra-toed grandparents. Here, as elsewhere, the absolute range of
variability depends upon the number of individuals observed.
TABLE 13.--_Distribution of toe-numbers in the offspring of
DR × R matings._
+--------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES |
+------+---------------------------------+------------------------------+--------------------------------+
| | Mother. | Father. | Offspring. |
| +----+----+-----------------+-----+----+-------------------+-----+-----+-----+-----+-----+--------+
|Serial|No. | | | No | | | No. | | | | |Average |
| No. |of | No.| Races involved. | of | No.| Races involved. | of | 4-4 | 4-5 | 5-5 | 4-6 |num. of |
| |pen.| | |toes.| | |toes.|toes.|toes.|toes.|toes.|toes per|
| | | | | | | | | | | | | bird. |
+------+----+----+-----------------+-----+----+-------------------+-----+-----+-----+-----+-----+--------+
| 1 |519A| 87|Houd. × Wh. Legh.| 4-5 | 71| Wh. Legh. | 4-4 | 17 | 2 | 6 | .. | 8.6 |
| 2 |671 | 742|Min. × Dk. Brah. | 4-4 | 352| Houd. × Dk. Brah. | 4-4 | 8 | 2 | 2 | .. | 8.5 |
| | | | +-----+-----+-----+-----+--------+
| | | | Totals (37) | 25 | 4 | 8 | .. | 8.54 |
+------+----+----+------------------------------------------------------+-----+-----+-----+-----+--------+
| B. SILKIE CROSSES. |
+------+----+----+-----------------+-----+----+-------------------+-----+-----+-----+-----+-----+--------+
| 3 |706 | 10|Wh. Legh. | 4-4 |1965| Silkie × Spanish | 5-5 | 4 | .. | 4 | .. | 9.00 |
| 4 |766 |3814| Do. | 4-4 | 834| Blk. Game × Silkie| 5-5 | 10 | 4 | 8 | 1 | 9.00 |
| 5 |766 | 10| Do. | 4-4 | 834| Do. | 5-5 | 7 | .. | 5 | .. | 8.83 |
| 6 |607 | 203|Frizzle × Silkie | 5-5 | 15| Frizzle | 4-4 | 15 | 2 | 9 | .. | 8.77 |
| 7 |766 |3815|Wh. Legh. | 4-4 | 834| Blk. Game × Silkie| 5-5 | 11 | .. | 7 | .. | 8.77 |
| 8 |706 |3815| Do. | 4-4 |1965| Silkie × Spanish | 5-5 | 6 | .. | 3 | .. | 8.67 |
| 9 |706 | 71| Do. | 4-4 |3823| Do. | 5-5 | 18 | 1 | 8 | .. | 8.63 |
| 10 |766 |3832|Buff Legh. | 4-4 | 834| Blk. Game × Silkie| 5-5 | 7 | .. | 2 | .. | 8.44 |
| 11 |706 |3833| Do. | 4-4 |1965| Silkie × Spanish | 5-5 | 3 | 1 | | .. | 8.25 |
| 12 |607 | 230|Frizzle × Silkie | 4-4 | 15| Frizzle | 4-4 | 23 | 2 | 2 | .. | 8.22 |
| 13 |706 | 71|Wh. Legh. | 4-4 |1965| Silkie × Spanish | 5-5 | 5 | .. | .. | .. | 8.00 |
| 14 |706 |3814| Do. | 4-4 |1965| Do. | 5-5 | 6 | .. | .. | .. | 8.00 |
| 15 |706 |3832|Buff Legh. | 4-4 |1965| Do. | 5-5 | 5 | .. | .. | .. | 8.00 |
| | | | +-----+-----+-----+-----+--------+
| | | | Totals (179) | 120 | 10 | 48 | 1 | 8.60 |
+------+----+----+------------------------------------------------------+-----+-----+-----+-----+--------+
TABLE 14.--_Distribution of toe-numbers in the offspring of DR × D
matings._
+-------------------------------------------------------------------------------------------------------+
| A. HOUDAN CROSSES. |
+------+---+---------------------------+-------------------------+--------------------------------------+
| | | Mother. | Father. | Offspring. |
| | +---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+
|Serial|Pen|No.| Races involved. | No. | No.|Races involved.| No.| 4-4 |4-5 | 5-5 | 5-6 | 6-6 |Average |
| No. |No.| | | of | | | of |toes.|toes.|toes.|toes.|toes.|num. of |
| | | | |toes.| | |toes| | | | | |toes per|
| | | | | | | | | | | | | | bird. |
+------+---+---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+ bird.
| 1 |803|529|Houdan × Min. | 5-5 |7522|Houdan | 5-5| 1 | 4 | 13 | .. | .. | 9.67 |
+------+---+---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+
| B. SILKIE CROSSES |
+------+---+---+-----------------+-----+----+---------------+----+-----+-----+-----+-----+-----+--------+
| 2 |606|182|Frizzle × Silkie.| 4-4 |775 |Silkie. | 6-6| .. | 3 | 10 | 3 | 5 | 10.48 |
| 3 |606|182| Do. | 4-4 | 21A| Do. | 6-6| .. | .. | 5 | .. | 1 | 10.33 |
| 4 |606|182| Do. | 4-4 |551 | Do. | 5-6| .. | .. | 5 | .. | .. | 10.00 |
| | | | +-----+-----+-----+-----+-----+--------+
| | | | Totals (32) | .. | .. | 20 | 3 | 6 | 10.36 |
+------+---+---+-------------------------------------------------+-----+-----+-----+-----+-----+--------+
TABLE 15.--_Percentages of the various types of toes in F1, F2,
DR × R and DR × D matings of the polydactyl crosses compared._
+--------+-----------------------------------+-----------------------------------+-----------------------------------+
| | | | _c._ Silkie crosses (reduced |
| | _a._ Houdan crosses. | _b._ Silkie crosses. | forms of toe classified as |
| No. of | | | typical). |
| toes. +--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+
| | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating | Mating |
| | F1. | F2. | DR × R | DR × D | F1. | F2. | DR × R | DR × D | F1. | F2. | DR × R | DR × D |
+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+
| |_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|_P. ct._|
| 4-4 | 27.3 | 47.4 | 67.6 | 5.6 | 9.4 | 31.7 | 67.0 | .. | 4.3 | 30.8 | 66.7 | .. |
| 4-5 | 19.1 | 12.1 | 10.8 | 22.2 | 9.4 | 7.7 | 5.6 | 9.4 | 2.9 | 3.8 | 3.1 | 9.4 |
| 5-5 | 53.6 | 39.7 | 21.6 | 72.2 | 81.2 | 51.4 | 26.8 | 62.5 | 76.8 | 53.4 | 24.6 | 62.5 |
| 4-6 | .. | .3 | .. | .. | .. | 1.0 | .6 | .. | .. | 1.0 | 1.9 | .. |
| 5-6 | .. | .5 | .. | .. | .. | 4.3 | .. | 9.4 | 5.8 | 5.8 | 1.5 | 9.4 |
| 6-6 | .. | .. | .. | .. | .. | 3.9 | .. | .. | 8.7 | 5.3 | 1.2 | 18.7 |
| 6-7 | .. | .. | .. | .. | .. | .. | .. | .. | .. | .. | .. | .. |
+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+
As we have seen, failure of dominance is much more complete in
some of the individuals of F2, namely, those with 4 toes, than
others. There is a variation in "potency." Is the degree of potency
inherited? Do the 4-toed heterozygotes produce a larger proportion of
imperfect dominants in F2 than the 5-toed heterozygotes? The answer
to this question should be given by the correlation between total
number of toes in the two parents and average number of toes in their
offspring, as given in table 11. In the case of the Houdan crosses
there is a strong positive correlation, measured by 0.683±0.092;
but the correlation is insignificant in the Silkie crosses
(-0.085±0.032). This lack of correlation in the Silkie hybrids is
perhaps due to the heavy regression in toe-number characteristic
of the second hybrid generation. In general, there seems to be an
inheritance of potency.
It now remains to test our conclusions by reference to the mating
of the heterozygote with the dominant and with the recessive types,
respectively. An examination of tables 13 to 15, particularly the
last, reveals several points of interest. Mendelian expectation
in the DR × R cross is 50 per cent of the recessive (4-4) type.
Actually, in the two crosses, A and B, 68 per cent and 67 per
cent, respectively, were obtained. But recalling that of these
amounts one-half of 27.3, or 13.71, and one-half of 9.4, or
4.7, are respectively due to failure to develop the extra-toe
in heterozygotes, there remain 54 per cent and 62 per cent,
respectively, of 4-toed offspring, which doubtless represent the
extracted RR type and approach the expected proportions.
Mendelian expectation in the DR × D cross (table 15) is 50 per
cent heterozygotes and 50 per cent extracted dominants. Of the
heterozygotes some 14 per cent may be expected to show 4-4 toes; that
the percentage is much less than that is doubtless due to the small
numbers involved. What is striking is the reappearance, in the second
generation, of large proportions of the extreme dominant type. These
results thus confirm those of the F2 generation.
Since extra-toe frequently fails to dominate, there should be certain
4-toed heterozygotes which throw extra-toe offspring, and such are
found. In table 16 are given six matings of 4-toed DR's. One sees
that they produce some 5-toed offspring. On the other hand, extracted
4-toed recessives are obtained, as table 17 shows.
Finally, we must consider whether, among the polydactyl birds of
one class, _e. g._, Houdans or Silkies, there is any difference
in the "centgener power" of parents corresponding to the degree
of development of their extra toes. This inquiry is suggested by
Castle's study (1906, p. 20) of polydactyl guinea-pigs. He finds that
when the extra toes of the mothers are graded into the 5 classes,
good (G), fair (F), poor (P), normal though of abnormal ancestry (N),
and normal of normal ancestry (N'), it follows: "first, that the
proportion of polydactylous young produced by a male decreases in
the successive classes from G to N'; and, secondly, that the degree
of development of the toes produced on those polydactylous young
diminishes in the same order." It is possible to test this conclusion
in poultry because, inside of any one type of extra-toe, _e. g._, the
triplex type, variation appears in the absolute size of the toes and
in the degree of their separateness. Our questions, then, are: (1)
does the _proportion of polydactyl young_ produced by a pair of birds
of any type diminish with the degree of development of toes inside
of that type, and (2) does the _degree of development_ of the toes
produced on the polydactylous offspring diminish in the same order?
TABLE 16.--_Distribution of toe-numbers in the offspring of
4-toed heterozygotes._
+-----+-------------------------+-----------------------+-----------------+--------+
| | Mother. | Father. | Offspring. | |
| Pen +----+-------------+------+----+-----------+------+-----+-----+-----+ Nature |
| No. | No.| Races. |No. of| No.| Races. |No. of| 4-4 | 4-5 | 5-5 | of |
| | | | toes.| | | toes.|toes.|toes.|toes.| mating.|
+-----+----+-------------+------+----+-----------+------------+-----+-----+--------+
| 637 | 685|Houd. × Min. | 4-4 | 570|Houd.×Min. | 4-4 | 5 | 1 | 2 |DR × DR |
| 729 | 913|Houd. × Min. | 4-4 | 936|Houd.×Legh.| 4-4 | 38 | 13 | 19 |DR × DR |
| 729 |2269| Do. | 4-4 | 936| Do. | 4-4 | 15 | 5 | 10 |DR × DR |
| 729 |2324| Do. | 4-4 | 936| Do. | 4-4 | 30 | 5 | 3 |DR × R |
| 642 | 750|Min. × Polish| 4-4 | 647| Do. | 4-4 | 10 | .. | 3 | R × DR |
| 671 | 742|Min. × Brah. | 4-4 | 352|Houd.×Brah.| 4-4 | 8 | 2 | 2 | R × DR |
+-----+----+-------------+------+----+-----------+------+-----+-----+-----+--------+
TABLE 17.--_Distribution of toe-numbers in the offspring of
extracted 4-toed parents._
+----+------------------------+--------------------------+-----------------+-------+
| | Mother. | Father. | Offspring. | |
| +----+-------------+-----+---+----------------+-----+-----+-----+-----+Nature |
|Pen | | | No. | | | No. | | | | of |
|No. | No.| Races. | of |No.| Races. | of | 4-4 |4-5 | 5-5 |mating.|
| | | |toes.| | |toes.|toes.|toes.|toes.| |
+----+----+-------------+-----+---+----------------+-----+-----+-----+-----+-------+
| {|2011|Polish × Min.| 4-4 |444|F2 Houd. × Legh.| 4-4 | 10 | .. | .. | R × R |
| {|2614| Do. | 4-4 |444| Do. | 4-4 | 6 | .. | .. | R × R |
|762{|2333| Do. | 4-4 |444| Do. | 4-4 | 16 | .. | .. | R × R |
| {|2618| Do. | 4-4 |444| Do. | 4-4 | 2 | .. | .. | R × R |
| {|3776| Do. | 4-4 |444| Do. | 4-4 | 2 | .. | .. | R × R |
+----+----+-------------+-----+---+----------------+-----+-----+-----+-----+-------+
Two sets of data are available for answering these questions. The
most direct set includes the data derived from crossing "pure-bred"
polydactyl birds and the other includes the data derived from using
hybrids between normal-toed and polydactyl ancestors. The latter data
have the advantage that the parents offer a greater variability;
but they have the disadvantage that the germinal condition of those
parents is incompletely known.
The pure races may be considered first. Eight matings of Houdans,
each parent with 5 toes, gave 122 offspring, of which 116 had 5-5
toes, 3 had 4-5 toes, and 3 had 4-4 toes. The variability of the toes
is not great in the parent Houdans. But, arranging them in the order
of development of the toes, the most developed first, the series of
table 18 results.
TABLE 18.
+------+---------+-------+--------------------------+
| | | | Offspring. |
|Serial| Pen No. |No. of +-----+-----+-----+--------+
| No. | |mother.| 4-4 | 4-5 | 5-5 |Average.|
| | | |toes.|toes.|toes.| |
+------+---------+-------+-----+-----+-----+--------+
| 1 | 727 803 | 2457 | 1 | 2 | 34 | 9.89 |
| 2 | 727 803 | 3105 | 1 | 0 | 45 | 9.95 |
| 3 | 803 | 2579 | .. | 1 | 12 | 9.92 |
| 4 | 727 | 3106 | .. | .. | 4 | 10.00 |
| 5 | 727 | 2494 | 1 | 0 | 5 | 9.67 |
| 6 | 727 | 2459 | .. | .. | 16 | 10.00 |
+------+---------+-------+-----+-----+-----+--------+
No direct relation here appears between development of the extra toe
in the parents and the average number of toes in the offspring.
Of the Silkies, 3 hens were used in 5 matings. The same 6-toed cock
(No. 774) was employed throughout (table 19).
TABLE 19.
+------+---------+----------+-------+---------------------------------+
| | | Mother. | | Offspring (No. of toes). |
|Serial| Pen No. +---+------+ f +---+---+---+---+---+---+---------+
| No. | |No.|No. of| |4-4|5-4|5-5|4-6|5-6|6-8| Average.|
| | | | toes.| | | | | | | | |
+------+---------+---+------+-------+---+---+---+---+---+---+---------+
| 1 | 734 815 |499| 6-6 |21{_a_ | 2 | 1 | 7 | 0 | 3 | 8 | 10.3 |
| | | | | {_b_ | 1 | 0 | 3 | 0 | 0 |17 | 11.4 |
| | | | | | | | | | | | |
| 2 | 734 815 |773| 6-5 |13{_a_ |.. |.. | 6 | 0 | 3 | 4 | 10.9 |
| | | | | {_b_ |.. |.. | 2 | 1 | 1 | 9 | 11.4 |
| | | | | | | | | | | | |
| 3 | 734 |500| 5-5 | 8{_a_ |.. | 2 | 4 | 0 | 2 |.. | 10.0 |
| | | | | {_b_ |.. |.. | 3 | 2 | 2 | 1 | 10.5 |
+------+---------+---+------+-------+---+---+---+---+---+---+---------+
In table 19 the series _a_ of observed average numbers of filial
toes (10.3, 10.9, 10.0) and the series _b_ obtained by assigning
the typical full number to all reduced types (11.4, 11.4, 10.5) are
decidedly irregular. There is, however, between the parental and the
filial series a correlation of +0.250±0.070. This indicates a slight
tendency for the number of toes in the progeny to vary with those of
the parentage.
The second set of data is derived from special matings made with
hybrids between Houdans and 4-toed races. On the one hand, in pens
728 and 813, cocks with well-developed toes of the duplex type were
mated with hens as nearly as possible of the same sort; while in
pens 765, 769, and 820 cocks with small, imperfectly separated toes
(probably of the duplex type[4]) were mated with hens as far as
possible of the same sort.
[4] I say probably of the duplex type because the cock
of pen 769 had a distally split toe on the right foot, reminding
somewhat of the _reduced_ triplex type. But as the left foot had a
typical duplex thumb, and the triplex is not common in Houdans, it
should probably be classed as duplex.
Tables 20, 21, and 22 give in detail and in summary the distribution
of types of polydactylism in the families from well-developed and in
those from poorly developed parents. They show a great difference
between the offspring of parents with good extra-toe (table 20) and
those with poor extra-toe (table 21). The former yield over 80 per
cent offspring with 5 toes or more on one or both feet, while the
latter yield about 57 per cent of such.
On the other hand, in the former families there are less than half
as many offspring with only 4 toes as in the latter. Classifying
"reduced" forms with their proper advanced type, we find highly
polydactyl parents yielding only 16 per cent non-polydactyl
offspring, while slightly polydactyl parents yield 43 per cent
non-polydactyl offspring. The percentage of polydactylous young
diminishes with the size and distinctness of the extra toes and the
grades of the polydactyl offspring are lower (absence in table 22, _b_,
of 6 toes). Both of Castle's conclusions seem to be confirmed.
TABLE 20.--_Distribution of toe-types in the offspring of "good"
extra-toed parents._
+------+---+---------------------------+---------------------------+---------+--------------------------------------+-------------------------------------------------------+
| | | Mother. | Father. | | Absolute numbers. | Theoretical classification. |
|Serial|Pen+----+----+-----------------+----+----+-----------------+ Mating. +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| No. |No.| No.|Gen.| Races. | No.|Gen.| Races. | | 4-4 | 4-5 | 5-5 | 5-6 | 6-6 |Average.| ss.| sd.|d'd'.|d'd.| dd.|d't'.|dt'.|dt.|t't'.|tt.|q't.|
+------+---+----+----+-----------------+----+----+-----------------+---------+-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| 1 |728|2271| F2 |Wh. Legh. × Houd.| 258| F1 |Houd. × Wh. Legh.| DD × DR | 4 | 1 | 21 | .. | .. | 9.65 | 3 | .. | 1 | 1 | 21 | .. | .. | ..| .. | ..| .. |
| 2 |728| 912| F2 | Do. | 258| F1 | Do. | DR × DR | 5 | 3 | 21 | .. | .. | 9.55 | 5 | 3 | .. | .. | 20 | .. | 1 | ..| .. | ..| .. |
| 3 |728|2248| F2 | Do. | 258| F1 | Do. | DD × DR | 8 | 3 | 22 | .. | .. | 9.42 | 8 | 3 | .. | .. | 21 | .. | .. | ..| 1 | ..| .. |
| 4 |728|2272| F2 | Do. | 258| F1 | Do. | DR × DR | 17 | 4 | 34 | .. | .. | 9.31 | 17 | 1 | .. | 3 | 34 | .. | .. | ..| .. | ..| .. |
| 5 |728| 174| F1 | Do. | 258| F1 | Do. | DR × DR | 10 | 1 | 15 | .. | .. | 9.19 | 10 | 1 | .. | .. | 14 | .. | 1 | ..| .. | ..| .. |
| | | | | +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| | | | | Totals (169) | 44 | 12 | 113 | .. | .. | 9.41 | 43 | 8 | 1 | 4 |110 | 0 | 2 | 0 | 1 | ..| .. |
| | | | | Percentages | 26.0| 7.1| 66.9| .. | .. | .. |25.4| 4.7| 0.6| 2.4|65.2| .. | 1.2| ..| 0.6 | ..| .. |
| | | | | +=====+=====+=====+=====+=====+========+====+====+=====+====+====+=====+====+===+=====+===+====+
| 6 |813|2271| F2 |Wh. Legh. × Houd.|3904| F3 |Houd. × Wh. Legh.| D × D | .. | 2 | 32 | .. | .. | 9.94 | .. | .. | .. | 2 | 32 | .. | .. | ..| .. | ..| .. |
| 7 |813|5113| F2 | Do. |3904| F3 | Do. | D × D | 2 | 1 | 32 | 1 | .. | 9.89 | .. | .. | 2 | 1 | 32 | .. | .. | 1 | .. | ..| .. |
| 8 |813| 377| F2 | Do. |3904| F3 | Do. | DR × D | 2 | 5 | 17 | .. | 1 | 9.68 | 2 | 2 | .. | 3 | 16 | .. | 1 | ..| .. | 1| .. |
| 9 |813|5122| F3 | Do. |3904| F3 | Do. | D × D | 1 | 3 | 7 | .. | .. | 9.55 | 1 | 3 | .. | .. | 7 | .. | .. | ..| .. | ..| .. |
| 10 |813| 935| F2 | Do. |3904| F3 | Do. | DR × D | 1 | 2 | 25 | 1 | 1 | 9.53 | 1 | 2 | .. | .. | 25 | .. | .. | 1 | .. | ..| 1 |
| 11 |813|2272| F2 | Do. |3904| F3 | Do. | DR × D | 5 | 2 | 18 | .. | .. | 9.52 | 4 | 1 | 1 | .. | 18 | 1 | .. | ..| .. | ..| .. |
| 12 |813| 912| F2 | Do. |3904| F3 | Do. | DR × D | 4 | 5 | 11 | .. | .. | 9.35 | 3 | 5 | 1 | .. | 11 | .. | .. | ..| .. | ..| .. |
| 13 |813|7320| F3 | Do. |3904| F3 | Do. | DR × D | 5 | 1 | 11 | .. | .. | 9.35 | 3 | 1 | 2 | .. | 11 | .. | .. | ..| .. | ..| .. |
| 14 |813|5142| F2 | Do. |3904| F3 | Do. | DR × D | 2 | 1 | 4 | .. | .. | 9.28 | 2 | .. | .. | 1 | 4 | .. | .. | ..| .. | ..| .. |
| | | | | +-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
| | | | | Totals (205) | 22 | 22 | 157 | 2 | 2 | 9.70 | 16 | 14 | 6 | 7 |156 | 1 | 1 | 2| 0 | 1| 1 |
| | | | | Percentages | 10.7| 10.7| 76.5| 1.0 | 1.0 | .. | 7.8| 6.8| 2.9| 3.4|76.2| 0.5 | 0.5|1.0| .. |0.5| 0.5|
+------+---+----+----+-------------------------------------------------------+-----+-----+-----+-----+-----+--------+----+----+-----+----+----+-----+----+---+-----+---+----+
TABLE 21.--_Distribution of toe-types in the offspring of "poor" extra-toed parents._
+------+---+----------------------------+----------------------------+--------+-------------------------------+------------------------------------+
| | | Mother. | Father. | | Absolute numbers. | Theoretical classification. |
|Serial|Pen+----+-----+-----------------+----+-----+-----------------+ Mating.+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| No. |No.| No.| Gen.| Races. | No.| Gen.| Races. | | 4-4 | 4-5 | 5-5 |5-6 |Average.| ss.| sd.|d'd'.|d'd.| dd.|t't'.|dq'.|
| | | | | | | | | | | | | | | | | | | | | |
+------+---+----+-----+-----------------+----+-----+-----------------+--------+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| 1 |765| 984|F2 |Wh. Legh. × Houd.|1794|F2 |Wh. Legh. × Houd.| DR × DR| 9 | 5 | 11 | .. | 9.08 | 9 | 3 | .. | 2 | 10 | 1 | .. |
| 2 |765|1790|F2 | Do. |1794|F2 | Do. | DR × DR| 18 | 7 | 17 | .. | 8.98 | 18 | 6 | .. | 1 | 17 | .. | .. |
| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| | | | | Totals (67) | 27 | 12 | 28 | .. | 9.02 | 27 | 9 | .. | 3 | 27 | 1 | .. |
| | | | | Percentages | 40.3| 17.9| 41.8| .. | .. |40.3|13.4| .. | 4.5|40.3| 1.5 | .. |
| | | | | +=====+=====+=====+====+========+====+====+====+====+====+=====+====+
| 3 |769| 492|F1 |Wh. Legh. × Houd.| 911|F2 |Wh. Legh. × Houd.| DR × DR| 13 | 1 | 14 | .. | 9.04 | 13 | 1 | .. | .. | 14 | .. | .. |
| 4 |769|4976|F2 | Do. | 911|F2 | Do. | DR × DR| 11 | 3 | 9 | .. | 8.91 | 11 | 3 | .. | .. | 8 | 1 | .. |
| 5 |769|2254|F2 | Do. | 911|F2 | Do. | DR × DR| 22 | 6 | 8 | .. | 8.61 | 22 | 4 | .. | 2 | 8 | .. | .. |
| 6 |769|1305|F2 | Do. | 911|F2 | Do. | DR × DR| 12 | 1 | 4 | .. | 8.53 | 12 | .. | .. | 1 | 4 | .. | .. |
| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| | | | | Totals (104) | 58 | 11 | 35 | .. | 8.77 | 58 | 8 | .. | 3 | 34 | 1 | .. |
| | | | | Percentages | 55.8| 10.6| 33.7| .. | .. |55.8| 7.7| .. | 2.9|32.7| 1.0 | .. |
| | | | | +=====+=====+=====+====+========+====+====+=====+====+====+=====+====+
| 7 |820| 984|F2 |Wh. Legh. × Houd.|4731|F3 |Wh. Legh. × Houd.| D × DR| 2 | 3 | 27 | .. | 9.78 | 2 | 2 | .. | 1 | 27 | .. | .. |
| 8 |820|2255|F2 | Do. |4731|F3 | Do. | DR × DR| 6 | 1 | 10 | .. | 9.24 | 6 | .. | .. | 1 | 10 | .. | .. |
| 9 |820|6479|F3 | Do. |4731|F3 | Do. | DR × DR| 12 | 2 | 16 | .. | 9.13 | 10 | 1 | 2 | 1 | 15 | 1 | .. |
| 10 |820|2016|F1[A]| Do. |4731|F3 | Do. | DR × DR| 9 | 2 | 2 | .. | 8.45 | 9 | 2 | .. | ..| 2 | .. | .. |
| | | | | +-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
| | | | | Totals (92) | 29 | 8 | 55 | .. | 9.28 | 27 | 5 | 2 | 3 | 54 | 1 | .. |
| | | | | Percentages | 31.5| 8.7 | 59.8| .. | .. |29.3| 5.4| 2.2| 3.3|58.7| 1.1 | .. |
+------+---+----+-----+-------------------------------------------------------+-----+-----+-----+----+--------+----+----+-----+----+----+-----+----+
[A] No. 2016 has 4-4 toes and is a hybrid between a 5-toed White Leghorn × Houdan and a 4-toed Minorca × Polish.
But a more critical examination of the parentages of the 5 pens shows
that they are not comparable. In matings 6 to 14 of table 20 the
cock is almost certainly a dominant in respect to toes; whereas the
cocks in table 21 are probably heterozygous. The heterozygous state
determines two things: the imperfect nature of the extra-toe and a
relative deficiency in the offspring of the higher toe-numbers. In
our results we can not say that one of these things is the cause of
the other, as Castle does; they are, rather, in all probability, due
to a common cause. I think Castle's paper may justly be criticized
for not giving sufficient data concerning the ancestry of the
individual mothers used. Without such data the paper can not be said
satisfactorily to demonstrate his conclusion.
TABLE 22.--_Summary of observed toe-numbers in offspring,
percentages._
+-------------------------------------------++-----------------------------+
| _a._ Parents have "good" extra toes. || _b._ Parents have "poor" |
| || extra toes. |
+--------+------+------+------+------+------++--------+------+------+------+
| | 4-4 | 4-5 | 5-5 | 5-6 | 6-6 || | 4-4 | 4-5 | 5-5 |
| Pen No.| toes.| toes.| toes.| toes.| toes.|| Pen No.| toes.| toes.| toes.|
+--------+------+------+------+------+------++--------+------+------+------+
| 728 | 26.0 | 7.1 | 66.9 | .. | .. || 765 | 40.3 | 17.9 | 41.8 |
| 813 | 10.7 | 10.7 | 76.5 | 1.0 | 1.0 || 769 | 55.8 | 10.6 | 33.7 |
| | | | | | || 820 | 31.5 | 8.7 | 59.8 |
| +------+------+------+------+------++ +------+------+------+
|Average.| 17.7 | 9.1 | 72.2 | 0.5 | 0.5 ||Average.| 43.2 | 11.8 | 44.9 |
+--------+------+------+------+------+------++--------+------+------+------+
To summarize: "Potency," as measured by dominance of the extra-toed
condition, is inherited, in the Houdan crosses at least. There is
some evidence, derived from "pure-bred" Silkies, that differences in
the degree of development of the extra-toes are inherited. But the
average condition of the toes in the offspring of second or later
generation hybrids can not be used as evidence of inheritance of the
degree of parental development of the toes, since these are dependent
on the same basal cause, namely, the hidden gametic constitution
of the parents. Despite the obscuration of imperfect dominance,
polydactylism in poultry proves itself to be a unit-character that
segregates.
CHAPTER III.
SYNDACTYLISM.
A. STATEMENT OF PROBLEM.
In man, various mammals, and some birds two or more adjacent fingers
are sometimes intimately connected by an extension of the web that is
normally a mere rudiment at their base. Such a condition is known as
syndactylism. A good introductory account of syndactylism is given by
Bateson (1904, pp. 356-358). Taking a number of cases of syndactylism
together, he says: "A progressive series may be arranged showing
every condition, beginning from an imperfect webbing together of the
proximal phalanges to the state in which two digits are intimately
united even in their bones, and perhaps even to the condition in
which two digits are represented by a single digit." He also calls
attention to the fact that in the human hand "there is a considerable
preponderance of cases of union between the digits III and IV;" while
in the foot the united digits "are nearly always II and III." The
matter of syndactylism in birds has a peculiar interest because of
the fact that among wading and swimming birds syndactylism has become
a normal condition of the feet, and, moreover, just this feature
is one that has become classical in evolutionary history, because
Lamarck thought it well illustrated his idea of the origin of an
organ by effort and use.
Concerning the cause of syndactylism little can be said. Both in
mammals and birds the digits are indicated before they are freed from
lateral tissue connections. The linear development of the fingers
is in part accompanied by a cutting back of this primordial web, in
part by a growth beyond it. In syndactylism growth of the web keeps
pace with that of the fingers. From this point of view syndactylism
may be regarded as due to a peculiar excessive development of the
web.[5] In some human cases adhesions of the apex of the appendage to
the embryonic membranes has stimulated the growth of the interdigital
membrane, resulting in syndactylism. But it would be absurd to
attempt to explain syndactylism in general on this ground. The more
"normal" forms of syndactylism, as seen in poultry, still want for a
causal explanation.
[5] Lewis and Embleton (1908, p. 45) present strong
arguments against the theory that syndactylism is due to
arrested development.
Most of the cases of syndactylism whose inheritance is about to be
described arose in a single strain of fowl and can, indeed, be traced
back to a single bird. This ancestor is No. 121, a Dark Brahma hen
described in a previous report.[6] It was only in the search for the
origin of the exaggerated forms of syndactylism observed in some of
her descendants that an unusually great extension of the web in her
feet was noticed. The syndactyl condition of my birds did not, thus,
arise _de novo_, but had its origin antecedent to the beginning of
the breeding experiments. In addition to this main strain a slight
degree of syndactylism has appeared among some of my Cochin bantams.
[6] Davenport, 1906, page 34, Plate V.
TABLE 23.--_Ancestry of syndactyl fowl and the results of various
matings involving syndactylism._
[Abbreviations: _Ab_α, _Ab_β, etc., types of syndactylism (p. 32);
F, father; FF, father's father; FM, father's mother; M, mother;
MF, mother's father; MM, mother's mother; M × P, hybrid of Minorca
and Polish races; Synd., syndactyl (type unknown). f, foot. In
Nos. 24 to 42 two cocks (Nos. 242 and 3116, and 5399 and 4562,
respectively) were at different times used.]
+-----------+-----+-------------------------------------------------------------++-------------------------------------------------------------------------+
| | | First mating. || Second mating. |
| | +-------------------------------------+-----------------------++--------------------------------------------+---------------+------------+
| | | Ancestry. | Offspring. || Ancestry. | Offspring. | Average |
| Serial | Pen +-----+--------+-------+-----+--------+--------+--------------++-----+--------+-------+-----+--------+------+---------------+ per |
| No. | No. | M's | | | F's | | | Syndactyl. || M's | | | F's | | | Syndactyl. | cent |
| | | No. | MM. | MF. | No. | FM. | FF. +----+---+-----+| No. | MM. | MF. | No. | FM. | FF. +----+----+-----+ syndactyl. |
| | | | | | | | |2f. |1f.| 0f. || | | | | | | 2f.| 1f.| 0f. | |
+-----------+-----+-----+--------+-------+-----+--------+--------+----+---+-----++-----+--------+-------+-----+--------+------+----+----+-----+------------+
| | | | | | | | | | | || | | | | | | | | | |
| 1_a_, _b_ | 627 | 302 | [1]121 | [2]8A | 180 | [1]121 | [2]8A | 0 | 0 | 34 || 302 | [1]121 | [2]8A | 242 | [1]121 |[2]8A | 3 | 0 | 29 | 10.3 |
| 2_a_, _b_ | 627 | 280 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 23 || 280 | 121 | 8A | 242 | 121 | 8A | 2 | 0 | 21 | 9.5 |
| 3_a_, _b_ | 627 | 181 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 20 || 181 | 121 | 8A | 242 | 121 | 8A | 3 | 0 | 33 | 9.1 |
| 4_a_, _b_ | 627 | 354 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 24 || 354 | 121 | 8A | 242 | 121 | 8A | 1 | 0 | 37 | 2.6 |
| 5_a_, _b_ | 627 | 178 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 20 || 178 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 42 | .. |
| 6_a_, _b_ | 627 | 190 | 121 | 8A | 180 | 121 | 8A | 1 | 0 | 24 || 190 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 6 | .. |
| 7_a_, _b_ | ... | 353 | 121 | 8A | 180 | 121 | 8A | 0 | 0 | 13 || 353 | 121 | 8A | 242 | 121 | 8A | 0 | 0 | 22 | .. |
| 8_a_, _b_ | ... | 300 | 121 | 1A | 180 | 121 | 8A | 0 | 0 | 23 || 300 | 121 | 1A | 242 | 121 | 8A | 0 | 0 | 37 | .. |
| +----+---+-----+| +----+----+-----+ |
| Totals (182) | 1 | 0 | 181 || Totals (236) | 9 | 0 | 227 | |
| Percentages |0.55| 0 |99.45|| Percentages |3.81| 0 |96.19| |
+----------------------------------------------------------------+----+---+-----++--------------------------------------------+----+----+-----+------------+
+------+------+--------------------------+--------------------------+--------------------------------------------------------------------------------------+
| | | Mother. | Father. | Offspring. |
|Serial| Pen +------+-------+-----------+------+-------+-----------+----------------------+---------------------------------------------------------------+
| No. | No. | | Bred | | | Bred | | Syndactyl. | Classification. |
| | | No. |in pen | Toes. | No. |in pen | Toes. +----+---+-----+-------+------------+------------+-----------+------------+------------+
| | | | No. | | | No. | | 2f.|1f.| 0f. | P. ct.| _Aa_α. | _Ab_α. | _Ab_β. | _Ab_β´. | _Bb_α. |
+------+------+------+-------+-----------+------+-------+-----------+----+---+-----+-------+------------+------------+-----------+------------+------------+
| 9 | 747 | 2526 |[3]658 |Normal. | 1888 |[3]658 |Normal. | 9 | 0 | 9 | 50.0 | .. | 2 | 16 | .. | .. |
| 10 | 747 | 2831 | 658 | Do. | 1888 | 658 | Do. | 6 | 0 | 6 | 50.0 | .. | 7 | 5 | .. | .. |
| 11 | 747 | 2652 | 658 | Do. | 1888 | 658 | Do. | 3 | 0 | 25 | 10.7 | .. | 6 | .. | .. | .. |
| 12 | 747 | 3541 | 658 | Do. | 1888 | 658 | Do. | 4 | 0 | 41 | 8.9 | 1 | 4 | 3 | .. | .. |
| 13 | 747 | 1892 | 658 | Do. | 1888 | 658 | Do. | 4 | 0 | 47 | 7.8 | .. | .. | .. | .. | .. |
| 14 | 747 | 1872 | 658 | Do. | 1888 | 658 | Do. | 0 | 0 | 28 | 0.0 | .. | .. | .. | .. | .. |
| 15 | 747 | 1874 | 658 | Do. | 1888 | 658 | Do. | 0 | 0 | 28 | 0.0 | .. | .. | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 26 | 0 | 184 | 12.4 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 16 | 703 | 2353 |D. Br. | Do. | 122 |D. Br. | Do. | 1 | 0 | 6 | 14.3 | .. | 2 | .. | .. | .. |
| 17 | 703 | 2030 |D. Br. | Do. | 122 |D. Br. | Do. | 2 | 1 | 12 | 20.0 | .. | 5 | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 3 | 1 | 18 | 18.2 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 18 | 754 | 3126 |[4]627 |Normal. | 871 |[4]627 |Normal. | 12 | 1 | 30 | 30.2 | .. | 13 | 12 | .. | .. |
| 19 | 754 | 3175 | 627 | Do. | 871 | 627 | Do. | 3 | 0 | 8 | 27.3 | .. | 3 | 3 | .. | .. |
| 20 | 754 | 873 | 627 | Do. | 871 | 627 | Do. | [2]|(?)| (?)| (?) | .. | .. | 4 | .. | .. |
| 21 | 754 | 1052 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 17 | 0.0 | .. | .. | .. | .. | .. |
| 22 | 754 | 853 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 19 | 0.0 | .. | .. | .. | .. | .. |
| 23 | 754 | 862 | 627 | Do. | 871 | 627 | Do. | 0 | 0 | 27 | 0.0 | .. | .. | .. | .. | .. |
| | | | | | | | +----+---+-----+-------+ | | | | |
| | | | | | | | | 15 | 1 | 101 | 13.7 | | | | | |
| | | | | | | | +====+===+=====+=======+ | | | .. | .. |
| 24 | 767 | 2526 |[3]658 |Normal. | 3116 |D. Br. | Synd. | 5 | 0 | 22 | 18.5 | 1 | 1 | 6 | .. | 2 |
| 25 | 767 | 872 |[5]627 |_Ab_β | 242 |[5]513 |Normal. | 1 | 0 | 1 | 50.0 | .. | 1 | .. | 1 | .. |
| 25_a_| 767 | 872 | 627 |_Ab_β | 3116 |D. Br. | Synd. | 7 | 1 | 30 | 21.0 | 3 | 5 | 3 | .. | 4 |
| 26 | 767 | 2104 |[7]608 |Normal. | 3116 |D. Br. | Do. | 3 | 0 | 18 | 14.3 | .. | 2 | 2 | .. | 2 |
| 27 | 767 | 2831 |[3]658 | Do. | 3116 |D. Br. | Do. | 3 | 0 | 32 | 8.6 | .. | 6 | .. | .. | .. |
| 28 | 767 | 181 |[6]513 | Do. | 242 | 513 |Normal. | 1 | 0 | 22 | 4.4 | 2 | .. | .. | .. | .. |
| 28_a_| 767 | 181 | 513 | Do. | 3116 |D. Br. | Synd. | 1 | 1 | 60 | 3.2 | .. | 1 | 1 | .. | 1 |
| 29 | 767 | 190 |[5]520 | Do. | 242 | 513 |Normal. | 1 | 1 | 28 | 6.7 | 1 | .. | .. | .. | 2 |
| 29_a_| 767 | 190 | 520 | Do. | 3116 |D. Br. | Synd. | 4 | | 49 | 7.6 | .. | 3 | 4 | .. | 1 |
| +----+---+-----+-------+ | | | | |
| Syndactyl (242 ♂) | 3 | 1 | 51 | 7.3 | | | | | |
| Syndactyl (3116♂) | 23 | 2 | 211 | 9.4 | | | | | |
+-------------------------------------------------------------------+----+---+-----+-------+------------+------------+-----------+------------+------------+
[1] No. 121 is a Dark Brahma.
[2] No. 8A is a Tosa fowl (Game).
[3] (White Leghorn × Rose Comb Black Minorca) × Dark Brahma.
[4] Dark Brahma.
[5] See _supra_.
[6] 121 ♂ Dark Brahma × 8A Tosa.
[7] F2 (White Leghorn × Dark Brahma).
TABLE 23.--_Ancestry of syndactyl fowl and the results of various
matings involving syndactylism_--Continued.
+------+-----+-----------------------------------------------------+-------------------------------------------------------------------------------------+
| | | Mother. | Father. | Offspring. |
|Serial| Pen +------+-------+-----------+------+-------+-----------+---------------------+---------------------------------------------------------------+
| No. | No. | | Bred | | | Bred | | Syndactyl. | Classification. |
| | | No. |in pen | Toes. | No. |in pen | Toes. +----+---+-----+------+------------+------------+-----------+------------+------------+
| | | | No. | | | No. | |2f. |1f.| 0f. |P. ct.| _Aa_α. | _Ab_α. | _Ab_β. | _Ab_β´. | _Bb_α. |
+------+-----+------+-------+-----------+------+-------+-----------+----+---+-----+------+------------+------------+-----------+------------+------------+
| 30 | 801 | 4569 | 767 |_Ab_α | 5399 | 747 |_Ab_α | 2 | 0 | 0 |100.0 | 1 | 0 | 3 | 0 | 0 |
| 30_a_| 801 | 4569 | 767 |_Ab_α | 4562 | 767 |Normal. | 0 | 2 | 2 | 50.0 | .. | 1 | 1 | .. | .. |
| 31 | 801 | 6843 | 767 | Normal. | 4562 | 767 | Do. | 1 | 3 | 2 | 66.7 | .. | 2 | 2 | 1 | .. |
| 32 | 801 | 872 | 627 |_Ab_β | 5399 | 747 |_Ab_α | 12 | 4 | 11 | 59.3 | 3 | 9 | 11 | .. | 5 |
| 32_a_| 801 | 872 | 627 |_Ab_β | 4562 | 767 |Normal. | 7 | 1 | 12 | 40.0 | 2 | 8 | 4 | 1 | .. |
| 33 | 801 | 5515 | 767 |_Bb_α | 5399 | 747 |_Ab_α | 4 | 0 | 7 | 36.4 | .. | 2 | 6 | .. | .. |
| 33_a_| 801 | 5515 | 767 |_Bb_α | 4562 | 767 |Normal. | 1 | 2 | 5 | 37.5 | 2 | 1 | 1 | .. | .. |
| 34 | 801 | 7528 | 767 |_Ab_β | 5399 | 747 |_Ab_α | 1 | 0 | 0 |100.0 | .. | 2 | .. | .. | .. |
| 34_a_| 801 | 7528 | 767 |_Ab_β | 4562 | 767 |Normal. | 2 | 1 | 7 | 30.0 | .. | 1 | 4 | .. | .. |
| 35 | 801 | 6861 | 767 |Normal. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 | .. | 2 | .. | .. | .. |
| 36 | 801 | 6869 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 3 | 25.0 | 1 | .. | .. | .. | .. |
| 36_a_| 801 | 6869 | 767 | Do. | 4562 | 767 |Normal. | 1 | 0 | 4 | 20.0 | .. | .. | 2 | .. | .. |
| 37 | 801 | 2831 | 658 | Do. | 5399 | 747 |_Ab_α | 3 | 1 | 18 | 18.2 | .. | 4 | .. | .. | 3 |
| 37_a_| 801 | 2831 | 658 | Do. | 4562 | 767 |Normal. | 2 | 1 | 11 | 21.4 | .. | 2 | .. | .. | 3 |
| 38 | 801 | 2526 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 5 | 0.0 | .. | .. | .. | .. | .. |
| 38_a_| 801 | 2526 | 658 | Do. | 4562 | 767 |Normal. | 1 | 0 | 2 | 33.3 | .. | 1 | 1 | .. | .. |
| 39 | 801 | 4570 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 5 | 16.7 | 1 | .. | .. | .. | .. |
| 39_a_| 801 | 4570 | 767 | Do. | 4562 | 767 |Normal. | 0 | 2 | 17 | 10.5 | 1 | 1 | .. | .. | .. |
| 40 | 801 | 1892 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 9 | 0.0 | .. | .. | .. | .. | .. |
| 40_a_| 801 | 1892 | 658 | Do. | 4562 | 767 |Normal. | 1 | 0 | 3 | 25.0 | .. | 2 | .. | .. | .. |
| 41 | 801 | 4263 | 767 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 4 | 20.0 | .. | 1 | .. | .. | .. |
| 41_a_| 801 | 4263 | 767 | Do. | 4562 | 767 |Normal. | 0 | 0 | 10 | 0.0 | .. | .. | .. | .. | .. |
| 42 | 801 | 6872 | 767 | Do. | 4562 | 767 | Do. | 0 | 0 | 6 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| Syndactyl (5399 ♂) | 22 | 8 | 62 | 32.6 | | | | | |
| Syndactyl (4562 ♂) | 17 |12 | 84 | 25.7 | | | | | |
| +====+===+=====+======+ | | | | |
| 43 | 776 | 2291 |Coch. |Normal. | 2732 |Coch. |Normal. | 2 | 0 | 6 | 25.0 | .. | 2 | .. | .. | 2 |
| 44 | 776 | 2574 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 9 | 10.0 | .. | 1 | .. | .. | .. |
| 45 | 776 | 2570 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 11 | 8.3 | .. | 1 | .. | .. | .. |
| 46 | 776 | 2297 |Coch. | Do. | 2732 |Coch. | Do. | .. | 1 | 12 | 7.7 | .. | .. | .. | .. | 1 |
| 47 | 776 | 2299 |Coch. | Do. | 2732 |Coch. | Do. | 1 | 0 | 16 | 5.9 | .. | 2 | .. | .. | .. |
| 48 | 776 | 2904 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 6 | 0.0 | .. | .. | .. | .. | .. |
| 49 | 776 | 2937 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 7 | 0.0 | .. | .. | .. | .. | .. |
| 50 | 776 | 2300 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 15 | 0.0 | .. | .. | .. | .. | .. |
| 51 | 776 | 2736 |Coch. | Do. | 2732 |Coch. | Do. | 0 | 0 | 18 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| | 3 | 3 | 100 | 5.7 | | | | | |
| +====+===+=====+======+ | | | | |
| 52 | 816 | 121 |D. Br.|_Ab_α | 122 |D. Br. |Normal. | 3 | 1 | 10 | 28.6 | .. | 1 | .. | 2 | 4 |
| 52_a_| 816 | 121 |D. Br.|_Ab_α | 4912 |M × P | Do. | 0 | 0 | 13 | 0.0 | .. | .. | .. | .. | .. |
| 53 | 816 | 5835 |D. Br.|Normal. | 122 |D. Br. | Do. | 1 | 0 | 6 | 14.3 | .. | 2 | .. | .. | .. |
| 54 | 816 | 2353 |D. Br.| Do. | 122 |D. Br. | Do. | 0 | 0 | 7 | 0.0 | .. | .. | .. | .. | .. |
| 54_a_| 816 | 2353 |D. Br.| Do. | 4912 |M × P | Do. | 0 | 0 | 4 | 0.0 | .. | .. | .. | .. | .. |
| +----+---+-----+------+ | | | | |
| Syndactyl ( 122 ♂) | 4 | 1 | 23 | 17.9 | | | | | |
| Syndactyl (4912 ♂) | 0 | 0 | 17 | 0.0 | | | | | |
+------------------------------------------------------------------+----+---+-----+------+------------+------------+-----------+------------+------------+
The types of syndactylism which have appeared in my flock form a
rather extensive series. First, (_A_) the single web, which, in my
specimens, always occupies the interspace between digits III and
IV. This is the same interval which is most apt to show the web in
syndactylism of the human hand, and, it is suggestive to note, it
is this interval that is filled in those wading birds that have the
single web only between the toes (_e.g._, _Cursorius_, _Glareola_,
_Vanellus_, _Squatarola_, _Charadrius_, _Limosa_, _Machetes_,
_Himantopus_); second, there is (_B_) the double web, one-seventh
as common, which always occupies the interspaces between the digits
II-III and III-IV.
On another basis, the syndactyl feet may be classified as: (_a_) toes
adherent, web small in extent, and (_b_) toes distant, web broad.
I have found the narrow web only between digits III and IV. It is
one-eighth as common as the broad-webbed type. The broad, double web
approaches closely to the type found normally in swans, geese, and
ducks.
Finally, the syndactyl feet may be classified as: α,
straight-toed, or β, curve-toed. Class α is
to class β in frequency as 2:1. In the typical curve-toed
syndactyl foot the web between III and IV is complete to the nails of
each; in fact, in extreme cases the nails of the two toes are more or
less fused together. From the fused nails the middle toe, being the
longer, passes in a curve to the distal end of the metatarsus. The
D-shaped interspace between the curved III and straight IV toe is
filled with the web. In other cases the nails are merely approximated
and the middle toe is slightly curved. In three instances (4 per cent
of all) the outer toe (IV) is curved toward the (straight) median toe
(class β´).
As stated, the polydactyl offspring trace back their ancestry to
No. 121; her feet both show the double, broad, straight-toed type
(_Bb_α). We shall attempt in the following paragraphs to
trace the heredity of her type of polydactylism and of the others
that have subsequently arisen.
B. RESULTS OF HYBRIDIZATION.
In taking up the results of breeding experiments to test the method
of inheritance of syndactylism, it will be best first to give in
a table all pens in which the character showed itself, with the
frequency of the different types of foot in them (table 23).
The history of the syndactyl strain begins with No. 121 ♀ and
in the matings 1 to 8 are given the results of crossing together some
of her progeny derived from a normal-toed father. This father was
either No. 8A or 1A, both full-blooded Tosa (Japanese Game) fowl and
without suspicion in either soma or offspring of syndactyl taint.
There is no record of trace of syndactylism in the progeny of 121 ×
8A (or 1A); but a slightly developed condition of syndactylism may
very well have been overlooked by me in this F1 generation (as I had
never thought of such an abnormality), even as I at first overlooked
the syndactylism visible in No. 121. But when these F1 hybrids were
mated together (pen 627, serial Nos. 1 to 8) I got, in the different
families, from 10 per cent syndactyl offspring down to none at all.
At first sight the suggestion arises that, if inheritance is at
all Mendelian, the normal condition is dominant and that the
heterozygotes throw again, in pen 627, the syndactylous condition.
If this hypothesis were true it would follow that syndactyls bred
together should, sometimes at least, throw, even in large families,
100 per cent syndactyl offspring. But only 2 families, Nos. 30 and
34, have yielded 100 per cent syndactyls, and these contained 2 and
1 offspring, respectively; so they are not significant. On the other
hand, there are numerous matings of 2 extracted normal-toed parents
that have produced only normal-toed offspring (families Nos. 14, 15,
21, 22, 23, including 119 individuals). Consequently the conclusion
is favored that normal-foot is recessive and syndactyl-foot dominant,
and this shall be our working hypothesis.
On our hypothesis, No. 121 is probably a heterozygote. Mated with the
recessive normal, expectation is 50 per cent heterozygous, showing
syndactylism; the remainder normal-toed. But dominance is here, as
in polydactylism, very imperfect. For this reason and because it
was not looked for, no syndactylism was noted in the first hybrid
generation. The offspring prove to be of two sorts, however. No. 180
♂ is a pure recessive, and in 8 matings with as many different
sisters of his he got 184 normal-toed to 1 syndactyl. These same
sisters, mated to another brother, No. 242, in some cases gave 9 per
cent and 10 per cent syndactyl. No. 242 is, consequently, probably
a DR and, mated to DR sisters (which constitute according to
expectation about one-half of all) gives some DD's, part of which
constitute the 9 to 10 per cent of syndactyls. Of course, 25 per
cent DD is to be expected; the difference gives a measure in this
instance of the imperfection of dominance in the "extracted" as well
as "heterozygous" condition.
Matings 9 to 15 (pen 747) are instructive in comparison with the
foregoing case. Both parents are derived from pen 658, which
contained as breeders a heterozygous Dark Brahma male (No. 146) and
various females of non-booted races far removed from suspicion of
syndactylism; expectation being an equal number of DR and RR
offspring. In pen 747 No. 1888 ♂ acts like a DR, and so do the
hens in matings 9 to 13, while the hens in the other 2 matings are
doubtless RR's. The former give 17 per cent syndactyl offspring,
the latter none at all (in 56 individuals).
Matings 16 and 17 (pen 703) are between pure-bred Dark Brahmas
that are probably DR's. About 22 per cent of their offspring are
syndactyl--a rather higher proportion than we have found before.
Matings 18 to 19 are between progeny of pen 627. In mating 20 the
normals were not recorded. The cock in this pen, No. 871, is probably
heterozygous, as are also the first two hens, so that nearly 30
per cent of their progeny are syndactyl. From the other 3 hens no
syndactyl offspring were obtained. Evidently the two sets of hens
have a very different gametic constitution. The existence of two
sorts of families is one of the strong arguments for the segregation
of this character.
We next come to the pens (matings Nos. 24 to 42) which were
especially mated to study the inheritance of syndactylism. I had now,
for the first time, two parents with syndactylic feet.
On account of imperfection of dominance decision as to gametic
composition of any parent must largely rest on the make-up of the
progeny. Table 24 gives the most reasonable classification of the
parentages.
TABLE 24.
+-----------------------------------------------------------------------------------+
| DD × DD (SYNDACTYL × SYNDACTYL). |
+------+--------+-------+-----------+--------+-------+-----------+------------------+
| | | | | | | | Syndactyl. |
|Family|Mother's|Bred in| Toes. |Father's|Bred in| Toes. +---+---+---+------+
| No. | No. |pen No.| | No. |pen No.| |2t.|1t.|0t.|P. ct.|
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 30 | 4569 | 767 |_Ab_α | 5399 | 747 |_Ab_α | 2 | 0 | 0 | 100.0|
| 34 | 7528 | 767 |_Ab_β | 5399 | 747 |_Ab_α | 1 | 0 | 0 | 100.0|
| 32 | 872 | 627 |_Ab_β | 5399 | 747 |_Ab_α |12 | 4 |11 | 59.3 |
| 33 | 5515 | 767 |_Bb_α | 5399 | 747 |_Ab_α | 4 | 0 | 7 | 36.4 |
| +---+---+---+------+
| Totals |19 | 4 |18 | 74.2 |
+----------------------------------------------------------------+---+---+---+------+
| DD × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 31 | 6843 | 767 |Normal. | 4562 | 767 |Normal. | 1 | 3 | 2 | 66.7 |
| 30_a_| 4569 | 767 |_Ab_α | 4562 | 767 | Do. | 0 | 2 | 2 | 50.0 |
| 33_a_| 5515 | 767 |_Bb_α | 4562 | 767 | Do. | 1 | 2 | 5 | 44.4 |
| 32_a_| 872 | 627 |_Ab_β | 4562 | 767 | Do. | 7 | 1 |12 | 42.9 |
| 34_a_| 7528 | 767 |_Ab_β | 4562 | 767 | Do. | 2 | 1 | 7 | 30.0 |
| 36 | 6869 | 767 |Normal. | 5399 | 747 |_Ab_α | 0 | 1 | 3 | 25.0 |
| 25_a_| 872 | 627 |_Ab_β | 3116 | D. Br.|Synd. | 7 | 1 |30 | 21.1 |
| 41 | 4263 | 767 |Normal. | 5399 | 747 |_Ab_α | 0 | 1 | 4 | 20.0 |
| 37 | 2831 | 658 | Do. | 5399 | 747 |_Ab_α | 3 | 1 |18 | 18.2 |
| 39 | 4570 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 1 | 5 | 16.7 |
| 40 | 1892 | 658 | Do. | 5399 | 747 |_Ab_α | 0 | 0 | 9 | 0.0 |
| +---+---+---+------+
| Totals |21 |14 |97 | 26.5 |
+----------------------------------------------------------------+---+---+---+------+
| DR × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 38_a_| 2526 | 658 | Normal. | 4562 | 767 | Normal. | 1 | 0 | 2 | 33.3 |
| 35 | 6861 | 767 | Do. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 |
| 40_a_| 1892 | 658 | Do. | 4562 | 767 | Do. | 1 | 0 | 3 | 25.0 |
| 37_a_| 2831 | 658 | Do. | 4562 | 767 | Do. | 2 | 1 |11 | 21.4 |
| 36_a_| 6869 | 767 | Do. | 4562 | 767 | Do. | 1 | 0 | 4 | 20.0 |
| 24 | 2526 | 658 | Do. | 3116 |D. Br. | Synd. | 5 | 0 |22 | 18.5 |
| 26 | 2104 | 608 | Do. | 3116 | Do. | Do. | 3 | 0 |18 | 14.3 |
| 39_a_| 4570 | 767 | Do. | 4562 | 767 | Do. | 0 | 2 |17 | 10.5 |
| 27 | 2831 | 658 | Do. | 3116 |D. Br. | Do. | 3 | 0 |32 | 8.6 |
| 29_a_| 190 | 520 | Do. | 3116 |D. Br. | Do. | 4 | 0 |49 | 7.6 |
| 29 | 767 | 190 | Do. | 242 | 513 | Do. | 1 | 1 |28 | 6.7 |
| 28_a_| 181 | 513 | Do. | 3116 | Do. | Do. | 1 | 1 |60 | 3.2 |
| +---+---+---+------+
| Totals |23 | 5 |249| 10.1 |
+----------------------------------------------------------------+---+---+---+------+
| RR × DR. |
+------+--------+-------+-----------+--------+-------+-----------+---+---+---+------+
| 42 | 6872 | 767 |Normal. | 4562 | 767 |Normal. | 0 | 0 | 6 | 0.0 |
| 41_a_| 4263 | 767 | Do. | 4562 | 767 | Do. | 0 | 0 |10 | 0.0 |
| +---+---+---+------+
| Totals | 0 | 0 |16 | 0.0 |
+----------------------------------------------------------------+---+---+---+------+
Summarizing the foregoing, and comparing the totals with Mendelian
expectation, we get the result shown in table 25.
A comparison of realization and expectation in table 25 shows that
the proportion of syndactyls is always less than expectation, not
only for dominants and heterozygotes together, but even for pure
dominants alone. The proportion of syndactyls obtained diminishes,
to be sure, in accordance with expectation (on the assumption that
they are pure dominants), but the numbers lag behind, in the higher
proportions 40 to 25 per cent. So we reach the conclusion that, as
in polydactylism, so in syndactylism dominance is very imperfect.
But there is this difference, that in syndactylism dominance is
so imperfect that the dominant condition rarely shows itself in
heterozygotes and even fails in many pure dominants. The striking
fact, the one that assures us the segregation is nevertheless
occurring in this case too, is that some families (whose two parents
are extracted recessives) throw 100 per cent recessives.
TABLE 25.
+-------------+-----+-----------------------------+--------------+
| | | Expectation. | Realization. |
| Nature of | +--------------+--------------+--------------+
| mating. | f | Dominants + | Pure | |
| | |heterozygotes.| dominants. | Syndactyls. |
+-------------+-----+--------------+--------------+--------------+
| | | _P. ct._ | _P. ct._ | _P. ct._ |
| DD × DD | 41 | 100.0 | 100.0 | 56.1 |
| DD × DR | 132 | 100.0 | 50.0 | 26.5 |
| DR × DR | 277 | 75.0 | 25.0 | 10.1 |
| RR × DR | 16 | 50.0 | 0.0 | 0.0 |
| RR × RR | 119 | 0.0 | 0.0 | 0.0 |
+-------------+-----+--------------+--------------+--------------+
These studies on syndactylism in poultry may be used for a critical
examination of the recent work of Lewis and Embleton (1908) on
syndactylism in man. The cases described by them follow the types I
have just described in poultry. Their fig. 18 corresponds to my types
_a_ and α; figs. 10 and 11 to my type β. The "crossbones"
referred to by the authors correspond to bones of the "curved toe."
The facts presented by the authors support the idea that syndactylism
is dominant rather than recessive, but they deny the application of
Mendelian principles to this case. Actually, the foot deformities
described by Lewis and Embleton are inherited much like syndactylism
in poultry. No extracted normal (recessive) extremity produces
the abnormal condition. Heterozygotes show much variation, from
very abnormal to slightly abnormal (possibly perfectly normal?)
appendages. Dominance is, indeed, much more potent than in poultry.
The authors' denial of the application of Mendelism to this case
seems to be based on an all too superficial consideration of the
hereditary behavior of the character and a tendency to "mass"
statistics--a procedure that tends to obscure the interpretation of
the data of heredity.
As to the inheritance of type, my statistics are not extensive enough
to give a final answer, but if all types be grouped into those with
straight and those with curved toes, then in crosses of straight-toed
syndactyl and normal 33 per cent of the offspring were of the curved
type, whereas in crosses of curved-toed syndactyls and normal 45 per
cent were of the curved type. These averages depend on 22 and 15
individuals, respectively. They lead us to look for an inheritance of
type when more extensive data shall be available.
Syndactylism is a typical sport, that is, a rather large mutation
having a teratological aspect. The question arises, Does it prove to
be prejudicial to the welfare of the species? The breeder who has
only a few individuals of a rare sport feels their loss more than
that of normals and the general impression left in his mind is that
the sport is less capable of maintaining itself than the normal form.
Assembling the data, consisting of about 40 individuals of each kind,
it appears that the death-rate is not very different in the two lots;
the slight excess of that of the syndactyls is sufficiently accounted
for by the circumstance that no _normals_ were reared during the
period of greatest mortality (the summer), but were destroyed or
given away as soon as hatched. It is probable, therefore, that
syndactylism, under the conditions of the poultry-yard, has little
life and death significance, but is one of those neutral characters
whose existence Darwin clearly recognized.
CHAPTER IV.
RUMPLESSNESS.
The tail of vertebrates is, historically, the post-anal part of the
trunk. Containing no longer any part of the alimentary canal, it has
lost much of its primitive importance, so that its disappearance in
any case is a matter of relatively little importance. Accordingly
we find groups of animals in which it is rudimentary or wholly
absent, such as many amphibia and the anthropoid apes and man. In
all recent birds the tail is a distinct but much reduced organ--the
uropygium--which contains several vertebræ in a degenerate condition.
The uropygium supports the tail feathers, which are of much use in
directing the bird in flight, but in ground birds, such as the grouse
and poultry, seem to function only for display in the male and, in
the female, to facilitate copulation.
Now, among various typically tailed vertebrates the tail is sometimes
absent. Tailless dogs, cats, sheep, and horses are known; on the
other hand, several cases of tails in man have been described
(Harrison, 1901). Thus the tail is a part of the body subject to
sporting; and it has also become the differential character for some
specific groups. In other words, it is an organ that has played
an important part in evolution and consequently its method of
inheritance is a matter of great interest.
The origin of the tailless poultry which I have bred has been
twofold. The most important strain is that referred to in an earlier
report[7] as Bantam Games. The second lot consists of rumpless fowl
that have arisen in my yards, spontaneously, from normal blood. Of
these more later.
[7] Davenport, 1906, pages 62 to 64, fig. 46.
The two rumpless Game cocks bore the numbers 117 and 116. Dr. A. G.
Phelps, of Glens Falls, New York, from whom the birds were purchased,
wrote that he had imported No. 117 from England, and No. 116 was its
son. The birds were very closely similar in all external features.
The matings made with No. 117 and their results are given in table 26.
TABLE 26.--_Progeny of tailless cock and tailed hens._
+------+------+--------+--------------------------+-----------------------------------+
| | | | Mother. | Offspring. |
| | | +----+---------------------+-------------------------+---------+
|Serial| Pen |Father's| | | Condition of uropygium. |Per cent |
| No. | No. | No. | No.| Races. |--------+------+---------|rumpless.|
| | | | | |Present.|Small.| Absent. | |
+------+------+--------+----+---------------------+--------+------+---------+---------+
| 1 |525 | 117 |114 | Nankin. | 3 | .. | 0 | 0 |
| 2 |526 | 117 | 20A| Frizzle. | 8 | .. | 0 | 0 |
| 3 |532 | 117 | .. |Bl. Coch. | 14 | .. | 0 | 0 |
| 4 |532_a_| 117 |127 |Wh. Legh. | 19 | .. | 0 | 0 |
| 4_a_|653 | 117 |508 |Bl. Coch. × Wh. Legh.| 8 | 3 | 0 | 0 |
| | | | | +--------+------+---------+---------+
| | | | | Totals | 52 | 3 | 0 | 0 |
+------+------+--------+----+---------------------+--------+------+---------+---------+
In 25 cases of the 52 an oil-gland was looked for and, in every case,
it was found to be missing.
Table 26, the conclusions from which were drawn in my 1906 report,
seemed to indicate the dominance of tail over its absence. On this
hypothesis I suspected that if No. 117 were bred to his (tailed)
offspring about 50 per cent of the progeny would be tailless, and if
the tailed hybrids of the F1 were bred together about 25 per cent of
their progeny should be tailless. The actual result of such matings
is shown in table 27.
TABLE 27.--_Heterozygotes mated with father._
+------+----+-------------------------------+-----------------------+
| | | Tailless cock × heterozygotes.| Offspring. |
| | +---------------+---------------+-----------------------+
|Serial| Pen| Father. | Mother. |Condition of uropygium.|
| No. | No.+-----+---------+-----+---------+--------+------+-------+
| | | No. | From | No. | From |Present.|Small.|Absent.|
| | | | pen No. | | pen No. | | | |
+------+----+-----+---------+-----+---------+--------+------+-------+
| 5 | 653| 117 |Original.| 577 | 532 | 6 | 1 | 0 |
| 6 | 653| 117 | Do. | 587 | 532 | 8 | 2 | 0 |
| 7 | 653| 117 | Do. | 635 | 532 | 7 | 0 | 0 |
| 8 | 653| 117 | Do. | 691 | 532 | 5 | 2 | 0 |
| 9 | 653| 117 | Do. | 652 | 532 | 15 | 0 | 0 |
| 10 | 653| 117 | Do. | 691 | 532 | 5 | 2 | 0 |
| 11 | 653| 117 | Do. | 705 | 532 | 9 | 2 | 0 |
| 12 | 653| 117 | Do. | 713 | 532 | 7 | 2 | 0 |
| 13 | 653| 117 | Do. | 760 | 532 | 13 | 2 | 0 |
| 14 | 653| 117 | Do. | 799 | 532 | 7 | 0 | 0 |
| | | +--------+------+-------+
| | | Total | 82 | 13 | 0 |
+------+----+-------------------------------+--------+------+-------+
TABLE 28.--_Heterozygotes mated inter se._
+------+----+----------+-----------+-----------------------------------------------+
| | | | | Condition of uropygium in offspring. |
| | | Father. | Mother. |-----------------------+-----------------------+
|Serial| Pen| | | Frequency. | Percentage. |
| No. | No.+----+-----+-----+-----+--------+------+-------+--------+------+-------+
| | | |From | |From | | | | | | |
| | | No.| pen | No. | pen |Present.|Small.|Absent.|Present.|Small.|Absent.|
| | | | No. | | No. | | | | | | |
+------+----+----+-----+-----+-----+--------+------+-------+--------+------+-------+
| 15 | 661| 466| 526 | 401A| 526 | 5 | 0 | 0 | 100 | 0 | 0 |
| 16 | 661| 466| 526 | 635 | 532 | 5 | 0 | 0 | 100 | 0 | 0 |
| 17 | 661| 466| 526 | 691 | 532 | 4 | 0 | 0 | 100 | 0 | 0 |
| 18 | 661| 466| 526 | 799 | 532 | 4 | 1 | 0 | 80 | 20 | 0 |
| 19 | 649| 516| 532A| 521 | 532A| 17 | 4 | 0 | 81 | 19 | 0 |
| 20 | 649| 516| 532A| 565 | 532A| 24 | 7 | 0 | 77 | 23 | 0 |
| 21 | 649| 516| 532A| 665 | 532A| 11 | 4 | 0 | 73 | 27 | 0 |
| 22 | 649| 516| 532A| 692 | 532A| 18 | 1 | 0 | 95 | 5 | 0 |
| 23 | 652| 343| 525 | 344 | 525 | 8 | 2 | 0 | 80 | 20 | 0 |
| 24 | 661| 428| 526 | 635 | 532 | 4 | 0 | 0 | 100 | 0 | 0 |
| 25 | 661| 428| 526 | 691 | 532 | 3 | 0 | 0 | 100 | 0 | 0 |
| 26 | 661| 428| 526 | 799 | 532 | 5 | 0 | 0 | 100 | 0 | 0 |
| +--------+------+-------+--------+------+-------+
| Total | 108 | 19 | 0 | 85 | 15 | 0 |
+----------------------------------+--------+------+-------+--------+------+-------+
The results given in tables 27 and 28 are remarkable. Neither in
the DR × R nor the DR × DR crosses did the tail fail to
develop. The tailless condition, that I had strongly suspected of
being recessive and expected in 25 per cent to 50 per cent of the
offspring, never once appeared. The only point of variation in the
uropygium of the chicks derived from the back cross or from F1's
bred _inter se_ was that in some the uropygium seemed distinctly
smaller than in the others. This small uropygium was as a matter
of fact recorded chiefly in chicks that failed to hatch, but it was
occasionally noticed in older birds, being then usually associated
with a slight convexity of the back. In some of the families the
uropygium is recorded as small in suspiciously close to 25 per cent
of the offspring. There is little doubt in my mind that this small
uropygium represents in some way the "absence" of tail that was
expected.
The next step was to cross the other rumpless bantam (No. 116),
to see if he behaved like his father. Accordingly, in pen 653, I
replaced the cock No. 117 by 116, the hens remaining the same, and
got the result shown in table 29.
TABLE 29.--_Heterozygotes mated with No. 116._
+------+------------+----------+----------------------------------------+
| | | | Condition of uropygium in offspring. |
|Serial| Father's | Mother's +----------+--------+---------+----------+
| No. | No. | No. | Present. | Small. | Absent. | Per cent |
| | | | | | | absent. |
+------+------------+----------+----------+--------+---------+----------+
| 27 | 116 | 508 | 5 | 2 | 10 | 59 |
| 28 | 116 | 577 | 3 | 0 | 3 | 50 |
| 29 | 116 | 587 | 3 | 1 | 4 | 50 |
| 30 | 116 | 652 | 4 | 0 | 2 | 33 |
| 31 | 116 | 705 | 3 | 1 | 5 | 56 |
| 32 | 116 | 713 | 1 | 0 | 2 | 67 |
| 33 | 116 | 760 | 4 | 0 | 2 | 33 |
| +----------+--------+---------+----------+
| Totals (55) | 23 | 4 | 28 | 51 |
+------------------------------+----------+--------+---------+----------+
Here we get a result almost exactly in accord with Mendelian
expectation. Having, now, obtained rumpless hens, it became possible
for the first time to test the inheritance of rumplessness in both
parents. The result is shown in the table 30.
TABLE 30.--_Rumpless fowl mated inter se._
+------+------+---------------+---------------+----------------------------+
| | | Father. | Mother. | Condition of tail in |
| | | | | offspring. |
| | +------+--------+------+--------+----------+--------+--------+
|Serial| Pen | | | | | | | |
| No. | No. | | From | | From | | | |
| | | No. | Serial | No. | Serial | Present. | Small. | Absent.|
| | | | No. | | No. | | | |
+------+------+------+--------+------+--------+----------+--------+--------+
| 34 | 742 | 2978 | 27 | 2601 | 29 | 0 | 0 | 4 |
| 35 | 854 | 2978 | 27 | 3430 | 27 | 0 | 0 | 9 |
| 36 | 742 | 2978 | 27 | 3430 | .. | [A]2 | 0 | 7 |
| 37 | 854 | 2978 | 27 | 2977 | 27 | [B]1 | 0 | 1 |
| +----------+--------+--------+
| Total | 3 | 0 | 21 |
+---------------------------------------------+----------+--------+--------+
[A] Both from chicks that died in shell.
[B] From a hatched chicken.
Table 30 is unfortunately small; one may say, fragmentary. Rumpless
hens are incapable of copulating unless the tail coverts are trimmed;
moreover my birds have been so much inbred that they are very weak;
finally, the chicks are so small that it is impracticable to rear
them in brooders and the eggs are particularly apt to be broken by
the brooding hens. However, it suffices to show that two tailless
fowl are able to throw some tailed offspring.
The second lot of rumpless fowl, namely, those that arose _de novo_
in my yards, must now be considered. In 1906, 2 birds hatched out
from ordinary tailed strains. As one was a cock and the other a
hen these were mated in 1907. The cock (No. 2464) came from No.
71♀ (a pure White Leghorn bred by myself from original White
Leghorn stock described in my 1906 report) and No. 235♂ (an F1
hybrid between one of these White Leghorns and my original Rose-comb
Black Minorca). The hen was No. 1636. Her mother (No. 618) was an F1
hybrid between a Minorca and Dark Brahma of series V, 1906 report,
and her father (No. 637) had the same origin. Thus the parents and
grandparents of both of these new rumpless birds were well known to
me and known to be fully tailed and to throw only tailed birds, with
the exception of these two birds.
The result of the mating of Nos. 2464 and 1636 in pen 736 was 25
chicks, of which 24 had tails and 1 (No. 5335) was without tail or
oil-gland. This, unfortunately, died early, so it was impossible to
breed it. In 1908, the hen No. 1636 having in the meantime died, I
mated No. 2464♂ to 6 of his (tailed) daughters. He was not well
and soon died, leaving no descendants by them, but 5 offspring by a
female cousin, all tailed. Then one of his sons (tailed) was mated
to its own sisters and produced 49 offspring, all tailed. Thus the
strain seems to have died out. The whole history is important both
because an apparently new mutation had taken place and because it
was, in a degree, "hereditary."
How, if at all, can this case and those of the bantams be brought
under known laws of inheritance? First of all, it must be confessed
that the provisional hypothesis, suggested in my earlier report, that
rumplessness is in my strain recessive has not been supported by the
newer facts. In the light of the principle of imperfect dominance
to which the facts of the last two chapters have led us, everything
receives a satisfactory explanation. The only conclusion that meets
all the facts is this: _The inhibitor of tail development--the
tailless factor--is dominant_; _its absence_--permitting a
continuation of the normal development of the tail region--_is
recessive_.
The application of this hypothesis to the various matings may now be
attempted. No. 117 is to be regarded as a heterozygote. The matings
with tailed birds is of the order DR × R, and expectation in the
typical case is 50 per cent DR (interrupted tail) and 50 per cent
RR (non-interrupted). But, owing to the relatively weak potency
of the interrupter derived from No. 117, growth of the tail is not
interrupted in the heterozygous offspring. These offspring are, by
hypothesis, so far as their gametes go, of two equally numerous
sorts, DR and RR. Mated to No. 117, two sorts of families are
to be expected, namely, the products of DR × RR (=50 per cent
DR, 50 per cent RR) and the products of DR × DR (=25 per
cent DD, 50 per cent DR, 25 per cent RR). The first lot of
families might be expected to resemble the preceding generation in
consisting entirely of tailed birds; the latter might be expected to
show in the 25 per cent extracted DD's evidence of the presence
of the undiluted interrupter. Actually in matings of the latter sort
(table 27) 3 families show no trace of the tail-interrupter, but in
7 there is evidence of a disturbance, as shown by the small size of
the uropygium and the bent back. In these families there are 13 cases
of small uropygium to 53 of large, being about 20 per cent of the
affected uropygium where 25 per cent was to be looked for--not a wide
departure, considering the liability of not recognizing the reduced
uropygium as such. This failure even of the extracted dominants
completely to stop the development of the tail gives a measure of the
weakness of the inhibitor in this case. Also, in table 28, matings
are varied. Some are probably matings of two heterozygotes, others of
two recessives, and others still of a recessive with a heterozygote.
On our hypothesis we should expect some of the families of the mated
hybrids to show evidence of the inhibiting factor and others to show
no such evidence. In those families in which small tail appears
it is found in about 19 per cent of the cases. On account of this
weakness of the inhibitor in the germ-plasm of No. 117 that inhibitor
is rarely fully activated. Only in one case out of the 250 or more
in which that germ-plasm is used is the development of the tail
completely stopped. In this case a hybrid cock derived from pen 526
(series 2, table 26) was crossed with various birds of tailed races
(probable RR's), and produced in addition to 20 tailed offspring 1
devoid of uropygium and oil-gland. In this case we may conceive that
an unusually potent condition of the inhibitor wholly stopped the
development of the tail.
The behavior of No. 116 is that of a pure dominant. Mated to
DR (and some RR?) females he produces pure dominants and
heterozygotes. His inhibiting factor is potent enough to be active in
the DD offspring at least; as a matter of fact 47 per cent of his
get have their tails inhibited. Even in the DR's the inhibitor may
sometimes work itself out. Thus No. 116 crossed on No. 508, without
tailless ancestry, had 56 per cent of the progeny without tail.
Since tailless birds may be either pure dominants or DR's, we may
expect families of two sorts when two such are bred together--those
containing only tailless offspring and those containing only 75 per
cent or less of such. Both sorts of families are to be expected in a
table with the composition of table 30, and both appear there.
The case of the rumpless fowl that arose _de novo_ will be explained,
then, as follows: Even in normal RR matings the inhibiting factor
may arise by mutation. But even when two of these inhibiting factors
are paired they show themselves so weak as not to appear in 25 per
cent, much less the typical 75 per cent of cases, but, as in our
case, merely 4 per cent. The strain takes on, indeed, the essential
features of the "eversporting varieties" of De Vries (1905). It seems
probable, therefore, that even in eversporting varieties inheritance
may be Mendelian, modified by variations in "potency" as shown by
irregularities in dominance.
CHAPTER V.
WINGLESSNESS.
The entire absence of appendages is a rare monstrosity, few cases
having been cited even for man. In my experience with poultry, out
of about 14,000 birds I have obtained one that had no wing on one
side of the body, but this unfortunately died before being bred from.
A second bird was given to me by a fancier. The bird was an Indian
Game, a vigorous cock, which was handicapped by his abnormality in
two ways. First, whenever he fell upon his side or back he was unable
to get upon his feet without aid. On several occasions he evidently
had spent hours upon the ground before he was discovered and picked
up. The wings are thus clearly most important to the fowl in enabling
it to regain its feet after having become prone. Secondly, he was
unable to tread a hen, since this act requires the use of wings as
balancers. He was, however, able to copulate with small birds without
leaving the ground. Thus in two respects his abnormality would have
proved fatal in nature. First, because of the personal risk, the
greater since a prone bird must fall an easy prey to predaceous
enemies; and secondly, because of the risk to his germ-plasm. Little
wonder, then, that this abnormality should not be known among wild
ground-birds.
Mated to 6 hens this wingless cock produced 130 chicks in 1907, of
which all had two wings. The following year he was mated to his
daughters, but died without leaving offspring. So I used a son of his
to mate with his own sisters and half-sisters. The progeny in this F2
generation consisted of 223 chicks, all of which had two wings. Thus,
no trace of winglessness appeared in any of the descendants of the
wingless cock.
The explanation of this case is not very certain, in view of the
limited data. It seems to resemble the behavior of No. 117, the
rumpless cock. And following the interpretation given in his case I
would conclude that winglessness is dominant to the normal condition,
that the original wingless cock was a heterozygote, and that the
dominance of winglessness was imperfect in the first generation. On
this hypothesis his son may well have been a pure recessive, and
then all of his descendants, in turn, would be either recessives
or heterozygotes (with imperfect dominance). It is, on the other
hand, possible that the wingless cock was a pure dominant, but that
the potency of the inhibitor was so slight as not to appear in the
heterozygotes or even in extracted dominants.
CHAPTER VI.
BOOTING.
The method of inheritance of the feathering on the feet of some
poultry has already been made the subject of much study. Hurst
(1905, p. 152) crossed booted and non-booted birds and bred the
hybrids together. He concluded that "the Mendelian principles are at
work in these aberrant phenomena, but are masked by something not
yet perceived." My own conclusion (1906, p. 72) was: "Booting is
dominant, but usually imperfectly so." A more extended study has been
desirable.
Booting is variable in amount. To indicate its degree I have had
recourse to an artificial scale. I recognize 11 grades, running from
0 to 10. The grade 0 implies no feathers whatsoever. Grade 10 implies
heavy booting extending over the front half of the shank. Grade 5
implies an extent of only half of the maximum, _i. e._, the outer
front quarter of the shank. Intermediate grades indicate intermediate
extension of the feathered area.
A. TYPES OF BOOTING.
The races of booted poultry used have been as follows: First, bantam
Cochins of two varieties; second, a bantam Dark Brahma; and third,
the Silkie. In my representatives of the first two groups, but
particularly in the Dark Brahma, the amount of booting is variable.
In one type the outer third of the shank in the newly hatched
chick is covered by strong, heavy, specialized feathers, directed
outward, while the middle and inner thirds are covered by smaller,
finer, imbricating feathers sparsely placed and resembling reduced
contour-feathers. In most individuals the transition from the one
kind to the other is gradual, while in others it is sharp, and in a
few the outer third only of the shank is feathered. In the Silkies,
which the standard poultry books describe as being more sparsely
feathered on the shank,[8] the outer zone of feathers is the only one
developed; and, occasionally, as table 31 shows, even these feathers
may be lacking. We have thus two types to distinguish--the extended
(Cochin, Brahma) type and the restricted type.
[8] Thus Wright (1902) says the shanks of the Silkies
(in England) are "slightly feathered," and Baldanus (1896) says
that (in Germany) they are feathered on the _outer half_.
B. NORMAL VARIABILITY.
To appreciate the results of hybridizing we must first examine the
variability of pure-blooded races. This is done in table 31.
TABLE 31.--_Distribution of boot-grades in the offspring of
Cochin, Dark Brahma, and Silkie parents._
+---------------------------------------------------------------------------------------------+
| A. OFFSPRING OF COCHIN PARENTS. |
+----+------------+------------+--------------------------------------------------------------+
| | Mother. | Father. | Grades of boot in offspring. |
|Pen |-----+------+------+-----+----+---+----+----+----+----+----+----+----+----+----+--------+
|No. | | Boot-| |Boot-| | | | | | | | | | | | |
| | No.| grade| No. |grade| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Average.|
+----+-----+------+------+-----+----+---+----+----+----+----+----+----+----+----+----+--------+
|848 | 2297| 10 | 545 | 10 |.. | ..| .. | 1 | .. | .. | 1 | .. | .. | 1 | 18 | 9.43 |
|776 | 2574| 10 | 2732 | 8 |.. | ..| .. | .. | .. | .. | .. | .. | 3 | 2 | 6 | 9.27 |
|848 | 2300| 8 | 545 | 10 |.. | ..| .. | .. | .. | .. | .. | .. | 1 | 2 | 5 | 9.25 |
|776 | 2570| 6 | 2732 | 8 |.. | ..| .. | .. | .. | .. | 1 | 1 | .. | 11 | 1 | 8.71 |
|848 | 2075| 9 | 545 | 10 |.. | ..| .. | .. | .. | 1 | 1 | .. | .. | .. | 4 | 8.50 |
|776 | 2072| 6 | 2732 | 8 |.. | ..| .. | .. | .. | .. | 1 | .. | 4 | 2 | 2 | 8.44 |
|758 | 130| 6 | 545 | 10 |.. | ..| .. | .. | .. | 1 | 1 | 1 | 3 | 9 | .. | 8.20 |
|776 | 2073| 6 | 2732 | 8 |.. | ..| .. | .. | 1 | 2 | .. | 2 | 2 | 10 | 1 | 8.00 |
|776 | 2300| 6 | 2732 | 8 |.. | ..| .. | .. | 1 | .. | 1 | 3 | 6 | 5 | 2 | 8.00 |
|758 | 131| 10 | 545 | 10 |.. | ..| .. | .. | .. | 1 | .. | 4 | 6 | 1 | 1 | 7.96 |
|776 | 2297| 6 | 2732 | 8 |.. | ..| .. | 1 | .. | 1 | .. | 3 | 6 | 6 | 2 | 7.95 |
|776 | 1132| 3 | 2732 | 8 |.. | ..| .. | 1 | 1 | 1 | 1 | 3 | 6 | 8 | .. | 7.57 |
|776 | 2937| 7 | 2732 | 8 |.. | ..| .. | .. | .. | .. | 1 | 3 | 3 | 1 | .. | 7.50 |
|776 | 2299| 7 | 2732 | 8 |.. | ..| 1 | .. | .. | 1 | 1 | 4 | 7 | 3 | 1 | 7.44 |
| +----|---+----+----+----+----+----+----+----+----+----+--------+
| Totals (199) |.. | ..| 1 | 3 | 3 | 8 | 9 | 24 | 47 | 61 | 43 | 8.24 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
| B. OFFSPRING OF DARK BRAHMA PARENTS. |
| [All individuals have sprung from No. 121 ♀ (boot of grade 9) and No. 122 ♂ |
| (boot of grade 6).] |
+---------------------------------------------------------------------------------------------+
|816 | 2030| 6 | 122 | 6 | .. | ..| .. | .. | .. | .. | .. | .. | .. | 1 | 3 | 9.8 |
|703 | 2030| 6 | 122 | 6 | .. | ..| .. | .. | .. | .. | 4 | 2 | 0 | 3 | 6 | 8.3 |
|816 | 121| 6 | 122 | 6 | .. | ..| .. | .. | .. | .1 | 3 | 1 | 2 | 4 | 5 | 8.3 |
|816 | 5979| 6 | 122 | 6 | .. | ..| .. | .. | .. | .. | 1 | 0 | 2 | .. | .. | 7.3 |
|816 | 2353| 5 | 122 | 6 | .. | ..| .. | .. |[A]1| 1 | 1 | 0 | 1 | 0 | 2 | 7.1 |
|816 | 5835| 5 | 122 | 6 | .. | ..| .. |[A]1| 0 | 1 | 2 | .. | .. | 1 | 3 | 6.5 |
|816 | 5840| 5 | 122 | 6 | .. | ..| .. |[A]1| .. | .. | 1 | .. | .. | .. | 1 | 6.3 |
|703 | 2353| 5 | 122 | 6 | .. | ..| .. | .. | 1 | 1 | 3 | .. | 1 | .. | .. | 5.8 |
| +----+---+----+----+----+----+----+----+----+----+----+--------+
| Totals (61) | .. | ..| .. | 2 | 2 | 4 | 15 | 3 | 6 | 9 | 20 | 7.62 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
| C. OFFSPRING OF SILKIE PARENTS. |
+----+-----+------+------+-----+----+---+----+----+----+----+----+----+----+----+----+--------+
|734 | 468| 4 | 774 | 3 | .. | ..| 1 | 2 | .. | .. | 1 | 1 | .. | .. | .. | 4.20 |
|734 | 1002| 3 | 774 | 3 | .. | ..| 1 | 4 | .. | 1 | 3 | .. | .. | .. | .. | 4.11 |
|734 | 841| (?)| 774 | 3 | .. | ..| .. | .. | 2 | .. | .. | .. | .. | .. | .. | 4.00 |
|815 | 7434| 7 | 774 | 3 | .. | ..| .. | .. | 2 | .. | .. | .. | .. | .. | .. | 4.00 |
|734 | 773| 1 | 774 | 3 | .. | ..| .. | 2 | 2 | .. | .. | .. | .. | .. | .. | 3.50 |
|734 | 680| 1 | 774 | 3 | .. | ..| .. | 2 | .. | .. | .. | .. | .. | .. | .. | 3.00 |
|734 | 405A| 1 | 774 | 3 | .. | ..| 1 | 3 | 1 | .. | .. | .. | .. | .. | .. | 3.00 |
|815 | 499| 2 | 774 | 3 | 1 | 1| 3 | .. | .. | 2 | .. | 1 | .. | .. | .. | 3.00 |
|734 | 499| 2 | 774 | 3 | 1 | 1| 5 | 2 | 2 | 1 | .. | .. | .. | .. | .. | 2.50 |
|734 | 500| 1 | 774 | 3 | 2 | 1| 2 | 3 | .. | .. | .. | .. | .. | .. | .. | 1.75 |
|815 | 773| 1 | 774 | 3 | 4 | 1| 3 | .. | .. | .. | .. | .. | .. | .. | .. | 1.25 |
|815 | 500| 1 | 774 | 3 | 1 | 1| .. | .. | .. | .. | .. | .. | .. | .. | .. | 0.50 |
|815 | 496| 3 | 774 | 3 | 1 | ..| .. | .. | .. | .. | .. | .. | .. | .. | .. | 0.00 |
| +----+---+----+----+----+----+----+----+----+----+----+--------+
| Totals(68) | 10 | 5| 16 | 18 | 9 | 4 | 4 | 2 | .. | .. | .. | 2.72 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
| SUMMARY. |
+------------------------------+--------------------------------------------------------------+
| | Grades of boot in offspring, reduced to percentages. |
| Races. +----+---+----+----+----+----+----+----+----+----+----+--------+
| | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Average.|
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
|Cochins | .. | ..| 0.5| 1.5| 1.5| 4.0| 4.5|12.1|23.6|30.7|21.6| 8.24 |
|Dark Brahmas | .. | ..| .. | 3.3| 3.3| 6.6|24.6| 4.9| 9.8|14.8|32.8| 7.62 |
|Silkie |14.8|7.4|23.5|26.5|13.2| 5.9| 5.9| 2.9| .. | .. | .. | 2.72 |
+------------------------------+----+---+----+----+----+----+----+----+----+----+----+--------+
[A] Determination made on embryo chicks.
An inspection of table 31 shows that, in respect to booting, the
Cochins and Dark Brahmas are clearly closely related to each other.
Owing to smaller numbers and to other circumstances that will be
discussed later, the results are less regular in the Dark Brahma
offspring, but in both the range is from 2 or 3 upward to 10, with
a great preponderance in grades above 5. In the Silkies, on the
other hand, the greatest frequency is found in grades below 5. This
difference is correlated with a difference of the parents, for the
commonest grades of the parents of the Cochins are between 6 and 10,
of the Dark Brahmas between 5 and 9, and of the Silkies between 1 and
3. These results suggest that the Silkie is typically heterozygous in
boot, producing 25 per cent recessives (boot of grade 4-7) and 75 per
cent dominant (0, 1) and heterozygous (2, 3). We shall see that this
hypothesis receives support from all Silkie matings.
Inside of any part of this table it appears that, on the whole, as
the average grade of the boot in the progeny diminishes that of
the parentage diminishes, although the correlation is by no means
perfect. Thus the average of the parental grades in the first part of
table 31, A (which is arranged in descending order of the averages
of the offspring) is 8.5; in the lower half, 7.4. The average of
parental grades in the upper half of table 31, B is 6.4; in the lower
half 5.5. In table 31, C the grades are 2.9 and 2.3, respectively.
This correlation indicates, without exactly measuring, heredity in
grade of booting.
Table 32 shows the results of crosses between Cochins (high grade of
boot) and Silkies (low grade).
TABLE 32.--_Distribution of boot-grades between a high and low
grade of boot in parents._
+---------------------------------------------------------------------------------------------------+
| HIGH AND LOW GRADE OF BOOT IN PARENTS. |
+---+-----------------------------+-----------------------+-----------------------------------------+
| | Mother. | Father. | Grade of boot in offspring. |
|Pen|-----+----+-------------+----+-----+----+-------+----+--+--+--+--+--+--+--+--+--+--+--+--------+
|No.| | | | | | | | | | | | | | | | | | | | |
| | No. |Gen.| Races. |Gra.| No. |Gen.| Race. |Gra.| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|Average.|
+---+-----+----+-------------+----+-----+----+-------+----+--+--+--+--+--+--+--+--+--+--+--+--------+
|851| 5567| P | Bl. × Bf. C.| 9 | 7526| P | Silkie| 3 |..|..|..|..| 2|..|..|..| 3| 3| 5| 8.15 |
|851| 3410| P | Do. | 9 | 7526| P | Do. | 3 |..|..|..|..|..| 4| 3| 2| 1| 6| 1| 7.29 |
|851| 6956| P | Do. | 8 | 7526| P | Do. | 3 |..|..|..|..| 3| 3|..| 2| 2|..| 5| 7.13 |
|851| 2073| P | Do. | 7 | 7526| P | Do. | 3 |..| 1|..| 1| 1|..| 1| 1| 1| 3| 2| 6.91 |
|851| 2299| P | Do. | 7 | 7526| P | Do. | 3 |..|..|..|..| 2| 2| 1| 1|..|..| 3| 6.78 |
|851| 840| P | Bf. C. | 10 | 7526| P | Do. | 3 |..|..|..|..| 1|..| 1|..|..| 1|..| 6.33 |
|851| 1002| P | Do. | 8 | 7526| P | Do. | 3 |..|..|..| 3| 1| 2| 1| 2| 4| 1| 1| 6.27 |
|815| 131| P | Bk. C. | 10 | 774| P | Do. | 4 |..|..|..| 3| 1| 1| 2| 2| 1| 1| 2| 6.23 |
|851| 841| P | Bf. C. | 10 | 7526| P | Do. | 3 |..|..|..|..| 1|..| 1|..| 1|..|..| 6.00 |
|851| 838| P | Do. | 8 | 7526| P | Do. | 3 |..|..|..| 4| 2| 4| 3|..|..| 2| 2| 5.65 |
| |--+--+--+--+--+--+--+--+--+--+--+--------+
| Totals (116) | 0| 1| 0|11|14|16|13|10|13|17|21| 6.77 |
+---------------------------------------------------------+--+--+--+--+--+--+--+--+--+--+--+--------+
So far as the average grade of boot in offspring goes, this table
stands between that of the Cochins (table 31, A) and that of the
Silkies (table 31, C). But what is especially striking is the
apparent dimorphism revealed in the line of totals. There is one
(empirical) mode at 10, corresponding with that of the Cochins, and
a second clear mode at 5, corresponding to that of the Silkies. If
we assume the Cochin to be homozygous in boot (RR) and the Silkie
to be heterozygous in boot, then we can interpret the high mode as
extracted recessives, the median mode as heterozygotes.
C. RESULTS OF HYBRIDIZATION.
We have next to consider the nature of the inheritance when one
parent belongs to an unbooted race, the other to a booted one (table
33).
TABLE 33.--_Distribution of boot-grades in the F1 generation of booted × non-booted parents._
+------------------------------------------------------------------------------------------------------------------+
| A. COCHIN CROSSES. |
+---+----------------------------+----------------------------+----------------------------------------------------+
|Pen| Mother. | Father. | Grade of boot in offspring. |
|No.|----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
| | No.|Gen.| Races. |Gra.| No. |Gen.| Races. |Gra.| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10|Average.|
+---+----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
|773|1334| P | W. Legh. | 0 | 836| P | Bl. Coch. | 10 | ..| ..| ..| 3| 1| 1| 1| 1| ..| 2| ..| 5.44 |
|773| 193| P | Do. | 0 | 836| P | Do. | 10 | ..| 1| 2| 6| 8| 7| 4| 2| ..| ..| ..| 4.27 |
|773|1366| P | Do. | 0 | 836| P | Do. | 10 | ..| ..| ..| 2| 5| 2| 1| ..| ..| ..| ..| 4.20 |
|773| 127| P | Do. | 0 | 836| P | Do. | 10 | ..| ..| 3| 10| 9| 12| 4| ..| ..| ..| ..| 4.11 |
|773| 692| P | W. Legh. (R)| 0 | 836| P | Do. | 10 | ..| ..| ..| 10| 3| 2| ..| ..| ..| ..| ..| 3.47 |
|774|2075| P | Coch. | 8 | 1431| P |W. Legh. (R)| 0 | 6| 1| 1| ..| 1| ..| ..| ..| ..| ..| ..| 0.78 |
| +---+---+---+---+---+---+---+---+---+---+---+--------+
| Totals (111) | 6| 2| 6| 31| 27| 24| 10| 3| 0| 2| 0| 3.91 |
+-------------------------------------------------------------+---+---+---+---+---+---+---+---+---+---+---+--------+
| B. DARK BRAHMA CROSSES. |
+---+----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
|727| Y| P | D. Br. | 10 | 381| P | Houd. | 0 | ..| ..| ..| ..| 2| 3| 2| 1| 2| ..| ..| 5.80 |
|727| 121| P | Do. | 10 | 381| P | Do. | 0 | 1| ..| ..| 1| 1| 5| 4| ..| ..| ..| ..| 4.67 |
|823|2030| P | Do. | 7 | 3858| P | M × P | 0 | ..| ..| 5| 16| 15| 4| 1| 2| ..| ..| ..| 3.67 |
|823| Y| P | Do. | 8 | 3858| P | Do. | 0 | ..| ..| 1| 7| 6| 2| ..| ..| ..| ..| ..| 3.56 |
|838|3814| P | W. Legh. | 0 | 122| P | D. Br. | 6 | ..| 2| 2| 6| 6| 1| 1| ..| ..| ..| ..| 3.28 |
|838| 202| P | Min. | 0 | 122| P | Do. | 6 | ..| ..| 2| 5| 3| ..| ..| ..| ..| ..| ..| 3.10 |
|838| 71| P | W. Legh. | 0 | 122| P | Do. | 6 | ..| ..| ..| 1| ..| ..| ..| ..| ..| ..| ..| 3.00 |
|838|3832| P | Do. | 0 | 122| P | Do. | 6 | 1| 1| ..| 1| 1| 2| ..| ..| ..| ..| ..| 3.00 |
|838| 10| P | Do. | 0 | 122| P | Do. | 6 | ..| 1| ..| 3| 1| ..| ..| ..| ..| ..| ..| 2.80 |
|816| 121| P | D. Br. | 9 | 4912| P | M × P | 0 | ..| ..| 8| 4| 1| 1| ..| ..| ..| ..| ..| 2.64 |
|816|5838| P | Do. | 9 | 4912| P | Do. | 0 | ..| ..| 5| 5| 1| ..| ..| ..| ..| ..| ..| 2.64 |
|838|5418| P | W. L., Min. | 0 | 122| P | D. Br. | 6 | 1| 1| 3| 3| 1| 1| ..| ..| ..| ..| ..| 2.50 |
|816|5979| P | D. Br. | 6 | 4912| P | M × P | 0 | 4| 3| 4| 7| 4| 1| 1| ..| ..| ..| ..| 2.46 |
|816|2353| P | Do. | 5 | 4912| P | Do. | 0 | ..| 2| 2| 4| 1| ..| ..| ..| ..| ..| ..| 2.44 |
|816|5977| P | Do. | 4 | 4912| P | Do. | 0 | ..| 3| 2| 1| ..| 1| ..| ..| ..| ..| ..| 2.14 |
|816|5835| P | Do. | 5 | 4912| P | Do. | 0 | 3| 5| 5| 8| 3| ..| ..| ..| ..| ..| ..| 2.12 |
|816|5840| P | Do. | 5 | 4912| P | Do. | 0 | 5| 1| 3| 4| 1| ..| ..| ..| ..| ..| ..| 1.64 |
|823|6626| P | Do. | 2 | 3858| P | Do. | 0 | 1| 10| 2| 2| ..| ..| ..| ..| ..| ..| ..| 1.33 |
|816|5980| P | Do. | 5 | 4912| P | Do. | 0 | 5| 8| 1| 5| ..| ..| ..| ..| ..| ..| ..| 1.32 |
| +---+---+---+---+---+---+---+---+---+---+---+--------+
| Totals (268) | 21| 37| 45| 83| 47| 21| 9| 3| 2| 0| 0| 2.84 |
+-------------------------------------------------------------+---+---+---+---+---+---+---+---+---+---+---+--------+
| C. SILKIE CROSSES. |
+---+----+----+-------------+----+-----+----+------------+----+---+---+---+---+---+---+---+---+---+---+---+--------+
|774| 777| P | Silkie. | 8 | 1176| P | W. Legh. | 0 | 3| ..| 1| 1| 1| ..| ..| ..| ..| ..| ..| 1.50 |
|744| 681| P | Do. | 5 | 1176| P | Do. | 0 | 11| 2| 1| 1| 1| 1| ..| ..| ..| ..| ..| 0.94 |
|744| 469| P | Do. | 1 | 1176| P | Do. | 0 | 11| 3| ..| ..| ..| ..| ..| ..| ..| ..| ..| 0.21 |
| +---+---+---+---+---+---+---+---+---+---+---+--------
| Totals (37) | 25| 5| 2| 2| 2| 1| 0| 0| 0| 0| 0| 0.76 |
+-------------------------------------------------------------+---+---+---+---+---+---+---+---+---+---+---+--------+
| SUMMARY. |
+---------------------------------------+---------------------------------------------------------------------------
| | Grades of boot in offspring, reduced to percentages. |
| Crosses. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
| | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Average.|
+---------------------------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
|Cochin | 5.4| 1.8| 5.4| 28.0| 24.3| 21.6| 9.0| 2.7| 0.0| 1.8| ..| 3.91 |
|Brahma | 7.8| 13.8| 16.8| 31.0| 17.5| 7.8| 3.4| 1.1| 0.7| ..| ..| 2.84 |
|Silkie | 67.6| 13.5| 5.4| 5.4| 5.4| 2.7| ..| ..| ..| ..| ..| 0.76 |
+---------------------------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
An inspection of Table 33, which gives the distribution of grades of
boot in the offspring constituting the first hybrid generation, might
well lead to the conclusion that inheritance is here of a blending
nature, or that, if either condition is dominant, it is the booted
one, as suggested in my report of 1906. On this hypothesis the
offspring with no boot illustrate imperfection of dominance, and one
would say that, in booting, dominance is very imperfect.
However plausible such an interpretation might appear when based
on the first hybrid generation alone, it becomes untenable when
subsequent generations are taken into account, as we shall see
later. The hypothesis breaks down completely in the second hybrid
generation and we are forced to the opposite hypothesis, namely, that
the clean-shanked condition is dominant. Such an hypothesis would
seem, at first, to contravene the principle enunciated in my report
of 1906 that the more progressive condition is dominant over the
less progressive condition, or absence. But such is not necessarily
the fact. We have no right to assume that presence of boot is the
new character. The rest of the body of poultry (save the head) is
covered with feathers. If the foot is not it must be because there
is something in the skin of the foot that inhibits the development
of feathers there. And this inhibiting factor is dominant over its
absence.
Table 33 shows that the Silkie crosses yield an exceptionally high
per cent of the dominant clear-footed condition. This is additional
evidence that the Silkies are DR, and so this cross produces 50
per cent of pure extracted dominants in addition to 50 per cent of
heterozygotes in booting.
To get further light on the nature of inheritance of booting we pass
to the examination of the second hybrid generation (table 34).
In the case of Silkies, which throw 67.6 per cent clean-shanked
progeny in F1, we find in F2 only about 60 per cent clean-shanked.
This diminution is, of course, due to the extraction of some pure
booted recessives, which draw from the proportion of clean shanks.
In the case of the Cochins and Dark Brahmas, expectation, with
perfect dominance, is that 75 per cent of the offspring shall
be clean-shanked. Since dominance is imperfect (as shown by the
occurrence of many booted birds in F1) we should look for an actual
failure to reach so large a proportion, but we are hardly prepared
for the result that in most of the F2 crosses of Cochins and Brahmas
less than 25 per cent of the offspring are clean-shanked. In 4 pens
the average is only 10 to 12 per cent, and in one only 2 per cent
of the offspring fail to develop feathers on the feet. What shall
we say of such a case as the last? The history of the father (No.
666) is absolutely certain; his mother was No. 121, the original
Dark Brahma female, with a boot of grade 9 and a record in her
immediate progeny that indicates perfect purity of booting in her
germ-cells. His father was a White Leghorn with clean shanks and
without a suspicion of having such antipodal blood as the Asiatic
in his ancestry. No. 666 is certainly heterozygous in boot, if
boot is a single unit. The hens with which No. 666 were mated were
clearly heterozygous, as is known not only from their ancestry, but
also from their behavior when mated with another cock, No. 254, in
which case they threw 12 per cent non-booted offspring. If now both
parents are heterozygous they must produce 25 per cent recessives.
This is the fact that forces us to conclude that clean shank is not
recessive, but dominant and due to an inhibitor that frequently
_fails to dominate_. In table 31 the two recessive varieties, mated
_inter se_, produce no featherless shanks; the feathers grow freely
as they do over the rest of the body. Some of the Silkies of table
31, however, are really heterozygous, with the dominant inhibitor not
showing; consequently they throw a large proportion of non-booted
offspring. In F1, as table 33 shows, the heterozygous offspring
have a reduced boot and perfect dominance--complete inhibition of
boot--in from 6 to 68 per cent. Dominance is most complete in the
Silkies, where, the feathering being feeble, the inhibitor has, as
it were, less to do in overcoming it. In F2 the expected 75 per cent
dominant is approached in the case of the Silkies (62 per cent and
59 per cent, respectively), but inhibition is very imperfect in the
Cochin and Brahma crosses, being reduced to between 25 and 2 per
cent. More proof that boot is due to the absence of a factor rather
than to its presence is found in this generation. If absence of boot
is recessive, then, combined with imperfection of dominance, _at
least_ 25 per cent of the offspring should be recessive and probably
a much larger proportion. The results in table 34 are absolutely
incompatible with this hypothesis, since, in one case, there are only
2 per cent that can not develop boot. Two extracted clean-footed
birds sometimes throw boot and sometimes not, and this result is to
be expected on the hypothesis that clean-footedness is dominant, but
two heavily booted birds can not transmit the boot inhibitor.
TABLE 34.--_Distribution of boot-grade in the F2 generation of
booted × non-booted poultry._
+------------------------------------------------------------------------------------------------------------------+
| COCHIN CROSSES. |
+---+---------------------------------------+-------------------------------------+--------------------------------+
| | Mother. | Father. | Offspring. |
+---+---+-----+-----------------------------+---+----+---------------------+------+--------+-------+-------+-------+
|Pen|No.| Gen.| Races. | Grade.|No.|Gen.| Races. |Grade.| Boot | Boot | Boot | P. ct.|
|No.| | | | | | | | |present.|slight.|absent.|absent.|
+---+---+-----+---------------------+-------+---+----+-----+---------------+------+--------+-------+-------+-------+
|650|170| F1 |Bl. Coch. × Wh. Legh.| Pr. |265| F1 |Bl. Coch. × Wh. Legh.| Pr. | 19 | 2 | 2 | 8.7 |
|650|263| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 36 | 2 | 2 | 5.0 |
|650|278| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 26 | 4 | 4 | 11.8 |
|650|361| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 24 | 2 | 9 | 25.7 |
|650|364| F1 | Do. | Pr. |265| F1 | Do. | Pr. | 39 | 5 | 3 | 6.4 |
| | | | +--------+-------+-------+-------+
| | | | Totals (179) | 144 | 15 | 20 | 11.1 |
| | | | | | | | |
| | | | |========|=======|=======|=======|
|654|602| F1 |Wh. Legh. × Bf. Coch.| Pr. |704| F1 |Wh. Legh. × Bf. Coch.| Pr. | 11 | 4 | 5 | 25.0 |
|654|828| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 7 | 11 | 0 | 0.0 |
|654|640| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 13 | 2 | 3 | 16.7 |
|654|696| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 8 | 5 | 8 | 38.1 |
|654|767| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 3 | 1 | 3 | 42.9 |
|654|697| F1 | Do. | Pr. |704| F1 | Do. | Pr. | 4 | 3 | 6 | 46.2 |
| | | | +--------+-------+-------+-------+
| | | | Totals (97) | 46 | 26 | 25 | 25.8 |
+---+---+-----+-------------------------------------------------------------------+--------+-------+-------+-------+
TABLE 34.--_Distribution of boot-grade in the F2 generation of
booted × non-booted poultry_--Continued.
+-------------------------------------------------------------------------------------------------------------------+
| DARK BRAHMA CROSSES. |
+---+---------------------------------------+-------------------------------------+---------------------------------+
| | Mother. | Father. | Offspring. |
|Pen+----+-----+---------------------+------+----+-----+------------------+-------+--------+-------+-------+--------+
|No.| No.| Gen.| Races. |Grade.| No.| Gen.| Races. |Grade. |Boot |Boot |Boot |P. ct. |
| | | | | | | | | |present.|slight.|absent.|absent. |
+---+----+-----+---------------------+------+----+----+-------------------+-------+--------+-------+-------+--------+
|608| 384| F1 |Wh. Legh. × Dk. Brah.| Pr. |409 |F1|Wh. Legh. × Dk. Brah.|Pr. | 36 | 5 | 3 | 6.8 |
|608| 248| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 32 | 5 | 4 | 9.8 |
|608| 249| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 39 | 11 | 13 | 20.6 |
|608| 395| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 20 | 11 | 10 | 24.4 |
|608| 385| F1 | Do. | Pr. |409 |F1| Do. |Pr. | 20 | 6 | 14 | 35.0 |
| | | | +--------+-------+-------+--------+
| | | | Totals (229) | 147 | 38 | 44 | 19.2 |
| | | | +========+=======+=======+========+
|659| 762| F1 |Wh. Legh. × Dk. Brah.| Pr. |375 |F1|Wh. Legh. × Dk. Brah.|Pr. | 18 | 4 | 1 | 4.4 |
|659| 503| F1 | Do. | Pr. |375 |F1| Do. |Pr. | 23 | 6 | 2 | 6.5 |
|659| 382| F1 | Do. | Pr. |375 |F1| Do. |Pr. | 10 | 2 | 1 | 7.7 |
|659| 250| F1 | Do. | Pr. |375 |F1| Do. |Pr. | 33 | 7 | 5 | 11.1 |
|659| 737| F1 | Do. | Pr. |375 |F1| Do. Pr. | 19 | 2 | 3 | 12.5 |
|659| 387| F1 | Do. | Pr. |375 |F1| Do. Pr. | 16 | 6 | 4 | 15.4 |
| | | | +--------+-------+-------+--------+
| | | | Totals (162) | 119 | 27 | 16 | 9.9 |
| | | | +========+=======+=======+========+
|655| 720| F1 |Wh. Legh. × Dk. Brah.| Pr. |666 |F1|Wh. Legh. × Dk. Brah.|Pr. | 5 | 2 | .. | 0.0 |
|655| 724| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 6 | 1 | .. | 0.0 |
|655| 728| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 3 | 1 | .. | 0.0 |
|655| 730| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 4 | .. | .. | 0.0 |
|655| 732| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 9 | .. | .. | 0.0 |
|655| 734| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 3 | .. | .. | 0.0 |
|655| 761| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 6 | 2 | .. | 0.0 |
|655| 800| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 1 | .. | .. | 0.0 |
|655| 721| F1 | Do. | Pr. |666 |F1| Do. |Pr. | 9 | 1 | 1 | 9.1 |
| | | | +--------+-------+-------+--------+
| | | | Totals (54) | 46 | 7 | 1 | 1.9 |
| | | | +========+=======+=======+========+
|655| 724| F1 |Wh. Legh. × Dk. Brah.| Pr. |254 |F1|Wh. Legh. × Dk. Brah.|Pr. | 3 | .. | .. | 0.0 |
|655| 734| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 12 | 1 | .. | 0.0 |
|655| 800| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 13 | .. | 1 | 7.1 |
|655| 720| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 12 | .. | 1 | 7.7 |
|655| 728| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 8 | 1 | 1 | 10.0 |
|655| 761| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 17 | 4 | 4 | 16.0 |
|655| 732| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 8 | 1 | 2 | 18.2 |
|655| 730| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 7 | .. | 2 | 22.2 |
|655| 721| F1 | Do. | Pr. |254 |F1| Do. |Pr. | 9 | .. | 3 | 25.0 |
| | | | +--------+-------+-------+--------+
| | | | Totals (110) | 89 | 7 | 14 | 12.7 |
| | | | +========+=======+=======+========+
|632| 742| F1 |Min. × Dk. Brah. | Pr. |637 |F1|Min. × Dk. Brah. |Pr. | 4 | 1 | 0 | 0.0 |
|632| 690| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 27 | 6 | 1 | 2.9 |
|632| 631| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 32 | 11 | 2 | 4.4 |
|632| 618| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 35 | 8 | 2 | 4.4 |
|632| 700| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 18 | 3 | 2 | 8.7 |
|632| 703| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 14 | 11 | 3 | 10.7 |
|632| 743| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 22 | 2 | 3 | 11.1 |
|632| 599| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 23 | 8 | 4 | 11.4 |
|632| 524| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 18 | 6 | 5 | 17.2 |
|632| 576| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 14 | 9 | 6 | 20.7 |
|632| 638| F1 | Do. | Pr. |637 |F1| Do. |Pr. | 8 | 2 | 6 | 37.5 |
| | | | +--------+-------+-------+--------+
| | | | Totals (316) | 215 | 67 | 34 | 10.8 |
+---+----+-----+------------------------------------------------------------------+--------+-------+-------+--------+
+---+-------------------------------+--------------------------------+----------------------------------------------+
| | Mother. | Father. | Boot-grade in offspring. |
|Pen+----+-----+----------------+---+----+-----+-----------------+---+--+--+--+--+--+--+--+--+--+--+--+-----+-------+
|No.| No.| Gen.| Races. |Gr.| No.| Gen.| Races. |Gr.| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|Aver-|P. ct. |
| | | | | | | | | | | | | | | | | | | | | age.|absent.|
+---+----+-----+----------------+---+----+-----+-----------------+---+--+--+--+--+--+--+--+--+--+--+--+-----+-------+
|801|2526| F1 |Min. × Dk. Brah.| 2 |5399| F1 |W. L. × Dr. Brah.| 8 |..|..|..|..| 1|..|..| 1|..|..| 1| 7.0 | 0.0 |
|801|2831| F1 | Do. | 4 |5399| F1 | Do. | 8 | 1| 1| 1| 4| 1| 7| 2| 2| 2|..| 2| 5.0 | 4.3 |
|801|1892| F1 | Do. | 3 |5399| F1 | Do. | 8 | 1| 1| 0| 1| 2|..| 1|..| 1| 1| 1| 5.0 |11.1 |
| | | | +--+--+--+--+--+--+--+--+--+--+--+--+--+-------+
| | | | Totals (35) | 2| 2| 1| 5| 4| 7| 3| 3| 3| 1| 4| 5.2 | 5.71 |
+---+----+-----+-----------------------------------------------------+--+--+--+--+--+--+--+--+--+--+--+-----+-------+
TABLE 34.--_Distribution of boot-grade in the F2 generation of
booted × non-booted poultry_--Continued.
+-------------------------------------------------------------------------------------------------------------------+
| SILKIE CROSSES. |
+---+-------------------------------+-------------------------------+-----------------------------------------------+
| | Mother. | Father. | Boot-grade in offspring. |
|Pen+----+-----+----------------+---+----+-----+----------------+---+---+--+--+--+--+--+--+--+--+--+--+-----+-------+
|No.| No.| Gen.| Races. |Gr.| No.| Gen.| Races. |Gr.| 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|Aver-|P. ct. |
| | | | | | | | | | | | | | | | | | | | | age.|absent.|
+---+----+-----+----------------+---+----+-----+----------------+---+---+--+--+--+--+--+--+--+--+--+--+-----+-------+
|709|1955| F1 |Silkie × Spanish| 5 |1578| F1 |Silkie × Spanish| 0 | 5| 1| 2| 1| 1| 1| 1| 0| 0| 0| 0| 1.92| 41.7 |
|753|1966| F1 |Silkie × Min | 0 |2573| F1 |Min. × Silkie | 0 | 19| 4| 2| 2|..| 2| 2| 2| 1|..|..| 1.71| 55.9 |
|709|1963| F1 |Silkie × Spanish| 7 |1578| F1 |Silkie × Spanish| 0 | 23| 6| 1| 6| 7|..|..|..|..|..|..| 1.26| 53.5 |
|753|2575| F1 |Silkie × Min | 0 |2573| F1 |Silkie × Min. | 0 | 15| 3| 7|..|..|..|..|..|..|..|..| 0.68| 60.0 |
|753|2071| F1 | Do. | 0 |2573| F1 | Do. | 0 | 23| 4| 6|..|..|..|..|..|..|..|..| 0.49| 69.7 |
|709|1453| F1 | Do. | 1 |1578| F1 |Silkie × Spanish| 0 | 24|11| 3|..|..|..|..|..|..|..|..| 0.45| 63.2 |
|709|2223| F1 |Silkie × Spanish| 0 |1578| F1 | Do. | 0 | 32| 7| 3|..|..|..|..|..|..|..|..| 0.31| 76.2 |
| | | | +---+--+--+--+--+--+--+--+--+--+--+-----+-------+
| | | | Totals (227) |141|36|24| 9| 8| 3| 3| 2| 1| 0| 0| 0.87| 62.2 |
| | | | |===+==+==+==+==+==+==+==+==+==+==+=====+=======+
|830|4082| F1 |Silkie × W. Legh| 2 |3947| F1 |Silkie × W. Legh| 1 | 11| 8|..| 7| 1|..|..|..|..|..|..| 1.22| 40.7 |
|830|4079| F1 | Do. | 0 |3947| F1 | Do. | 1 | 18| 7| 6| 3|..|..|..|..|..|..|..| 0.82| 53.0 |
|830|5379| F1 | Do. | 0 |3947| F1 | Do. | 1 | 18| 4| 5| 3|..|..|..|..|..|..|..| 0.77| 60.0 |
|830|4081| F1 | Do. | 0 |3947| F1 | Do. | 1 | 24| 6|10| 1|..|..|..|..|..|..|..| 0.71| 58.5 |
|830|5374| F1 | Do. | 0 |3947| F1 | Do. | 1 | 11| 3| 3| 1|..|..|..|..|..|..|..| 0.67| 61.1 |
|830|3946| F1 | Do. | 0 |3947| F1 | Do. | 1 | 19| 1|..|..| 1|..|..|..|..|..|..| 0.24| 90.5 |
| | | | |===+==+==+==+==+==+==+==+==+==+==+=====+=======+
| | | | Totals (170) |101|29|24|14| 2| 0| 0| 0| 0| 0| 0| 0.75| 59.4 |
+---+----+-----+----------------------------------------------------+---+--+--+--+--+--+--+--+--+--+--+-----+-------+
The distribution of table 35 is characterized by its large
variability. Although the numbers are small, there are evidences of
two modes, one between grades 3 and 6, and the other at from 8 to 10;
these evidently correspond to the modes of the typical Silkie and
the typical Cochin respectively or to DR and RR types of booting
respectively. The distribution of table 35 is additional evidence of
the heterozygous nature of the Silkie boot.
TABLE 35.--_Distribution of boot-grades in Silkie × Cochin crosses._
+---+------------------------------+--------------------------------+-----------------------------------------------------------+
| | Mother. | Father. | Boot-grades in offspring. |
|Pen+----+----+--------------+-----+------+----+--------------+-----+--+---+---+---+---+---+---+---+---+---+----+-----+---------+
|No.| No.|Gen.| Races. | Gra.| No. |Gen.| Races. | Gra.| 0| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Av. | P. ct. |
| | | | | | | | | | | | | | | | | | | | | | abs. |
+---+----+----+--------------+-----+------+----+--------------+-----+--+---+---+---+---+---+---+---+---+---+----+-----+---------+
|821|5925| F1 | Silk. × Coch.| 7 | 6095 | F1 | Silk. × Coch.| 7 |..| ..| ..| ..| 1 |.. | ..| 1 | 3 | 1 | 1 | 7.7 | 0.0 |
|821|7408| F1 | Do. | 4 | 6095 | F1 | Do. | 7 |..| ..| ..| 1| 2 | 2 | 3 | ..| 2 | 1 | 2 | 6.5 | 0.0 |
|821|7413| F1 | Do. | 3 | 6095 | F1 | Do. | 7 | 2| 0| 3| 1| 0 | 1 | 1 | 0 | 0 | 1 | 1 | 3.9 | 20.0 |
|821|7416| F1 | Do. | 5 | 6095 | F1 | Do. | 7 |..| ..| ..| 3| 1 | 0 | 4 | 0 | 3 | 3 | 2 | 6.8 | 0.0 |
|821|7417| F1 | Do. | .. | 6095 | F1 | Do. | 7 |..| ..| ..| ..|.. |.. | ..| 1 |.. | 1 | 4 | 9.3 | 0.0 |
|821|7418| F1 | Do. | 4 | 6095 | F1 | Do. | 7 |..| ..| 1| ..| 2 | 1 | 1 | 1 | 1 |.. | 1 | 5.8 | 0.0 |
|821|7423| F1 | Do. | 6 | 6095 | F1 | Do. | 7 |..| ..| ..| 1|.. | 2 | ..| 2 | 2 |.. | 2 | 7.0 | 0.0 |
|821|7428| F1 | Do. | .. | 6095 | F1 | Do. | 7 | 1| ..| ..| ..|.. | 1 | ..| ..| 1 |.. |.. | 4.3 | 33.3 |
|821|7429| F1 | Do. | 8 | 6095 | F1 | Do. | 7 |..| ..| ..| 1| 1 | 1 | ..| ..|.. | 1 | 1 | 6.2 | 0.0 |
| | | | +--+----+--+---+---+---+---+---+---+---+----+-----+---------+
| | | | Totals (77) | 3| 0| 4| 7| 7 | 8 | 9 | 5 |12 | 8 |14 |6.42 | 3.90 |
| | | | |\--------------------/|\------------------/| | |
| | | | | 29 | 48 | | |
+---+----+----+-----------------------------------------------------+----------------------+--------------------+-----+---------+
We are now in a position to consider the effect of back crosses
(table 36). The contrast between the totals in tables 36 and
37 is very great. The strict Mendelian expectation is: in the
DR × D crosses 50 per cent DD (clean-footed) and 50 per cent
heterozygous, which, with imperfect dominance, might be expected to
show foot-feathering. Actually about 46 per cent are clean-footed.
In the DR × R crosses expectation is that 50 per cent certainly
(the extracted recessives) and 50 per cent more possibly will have
the shanks feathered, on account of imperfect dominance of the
heterozygotes. Actually all have feathered feet. These statistics
thus confirm the view of the dominance of the inhibiting factor.
Were clean shank recessive, then the DR × R crosses must give
50 per cent clean-footed and probably over. The actual result, none
clean-footed, is not in accord with the latter assumption.
TABLE 36.--_Distribution of boot-grade in DR × D (non-booted)
crosses._
+---+-----------------------------------------+---------------------------+--------------------------------------+
| | Mother. | Father. | Boot-grade in offspring. |
| +---+-----+----------------------+--------+---+----+---------+--------+---------+--------+--------+----------+
|Pen| | | | | | | | | | | | |
|No.|No.| Gen.| Races. | Grade. |No.|Gen.| Race. | Grade. | Present.| Slight.| Absent.| Per cent.|
| | | | | | | | | | | | | present. |
+---+---+-----+----------------------+--------+---+----+---------+--------+---------+--------+--------+----------+
|653|508| F1 | Wh. Legh. × Bf. Coch.| Pr. |117| P. | Game. | 0 | 3 | 4 | 6 | 46.2 |
|653|508| F1 | Do. | Pr. |116| P. | Do. | 0 | 6 | 5 | 4 | 26.7 |
|653|577| F1 | R × Bf. Coch. | 3 |117| P. | Do. | 0 | 1 | 0 | 7 | 87.5 |
|653|577| F1 | Do. | 3 |116| P. | Do. | 0 | 1 | 3 | 2 | 33.3 |
|653|587| F1 | Do. | 1 |117| P. | Do. | 0 | 1 | 2 | 4 | 57.1 |
|653|587| F1 | Do. | 1 |116| P. | Do. | 0 | 3 | 3 | 2 | 25.0 |
|653|635| F1 | Do. | 3 |117| P. | Do. | 0 | .. | 1 | 6 | 85.7 |
|653|635| F1 | Do. | 3 |116| P. | Do. | 0 | 2 | 2 | 1 | 20.0 |
|653|652| F1 | Do. | 5 |117| P. | Do. | 0 | 5 | 8 | 4 | 23.5 |
|653|652| F1 | Do. | 5 |116| P. | Do. | 0 | 1 | 2 | 2 | 40.0 |
|653|691| F1 | Do. | Pr. |117| P. | Do. | 0 | 2 | 2 | 1 | 20.0 |
|653|705| F1 | Do. | 2 |117| P. | Do. | 0 | 3 | 2 | 5 | 50.0 |
|653|705| F1 | Do. | 2 |116| P. | Do. | 0 | 1 | 1 | 5 | 71.4 |
|653|713| F1 | Do. | Pr. |117| P. | Do. | 0 | .. | 0 | 4 | 100.0 |
|653|713| F1 | Do. | Pr. |116| P. | Do. | 0 | 1 | 1 | 3 | 60.0 |
|653|760| F1 | Do. | Pr. |117| P. | Do. | 0 | 2 | 2 | 6 | 60.0 |
|653|760| F1 | Do. | Pr. |116| P. | Do. | 0 | 0 | 3 | 2 | 40.0 |
|653|799| F1 | Do. | 3 |117| P. | Do. | 0 | 2 | 0 | 3 | 60.0 |
| | | | +---------+--------+--------+----------+
| | | | Total (143) | 34 | 42 | 67 | 46.9 |
| | | | |=======================================
|661|635| F1 | Bf. Coch. × Game. | Pr. |466| P. | Game. | 0 | 1 | .. | 2 | 66.7 |
|661|635| F1 | Do. | Pr. |428| P. | Do. | 0 | 2 | .. | 1 | 33.3 |
|661|691| F1 | Do. | Pr. |466| P. | Do. | 0 | 2 | .. | 2 | 50.0 |
|661|691| F1 | Do. | Pr. |428| P. | Do. | 0 | 2 | .. | 1 | 33.3 |
|661|799| F1 | Do. | Pr. |466| P. | Do. | 0 | 3 | .. | 2 | 40.0 |
|661|799| F1 | Do. | Pr. |428| P. | Do. | 0 | 4 | .. | 1 | 20.0 |
| | | | +---------+--------+--------+----------+
| | | | Total (23) | 14 | 0 | 9 | 39.1 |
| | | | Grand Total (166) | 48 | 42 | 76 | 45.8 |
+-------+-----+-----------------------------------------------------------+---------+--------+--------+----------+
TABLE 37.--_Distribution of boot-grade in DR × RR (booted) crosses._
+----+--------------------+-----+------------------------------+---------------------------------------------+----+
| | Mother. | | Father. | Boot-grade in offspring. | |
| +-----+-----+--------+-----+-----+--------+----------+----+---+---+---+---+----+----+----+----+----+----+----+
|Pen | | | | | | | | | | | | | | | | | | | |
|No. | | | | | | | | | | | | | | | | | | | |
| | No. | Gen.| Race. | Gr. | No. | Gen. | Race. | Gr.| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+----+-----+-----+--------+-----+-----+--------+----------+----+---+---+---+---+----+----+----+----+----+----+----+
|851 | 838 | P. | Cochin.| 8 |7526 |[A]F1 | Silkie. | 3 |.. |.. |.. | 3 | 2 | 4 | 3 | .. | .. | 2 | 2 |
|851 | 840 | P. | Do. | 10 |7526 | F1 | Do. | 3 |.. |.. |.. |.. | 1 | .. | 1 | .. | .. | 1 | .. |
|851 | 841 | P. | Do. | 10 |7526 | F1 | Do. | 3 |.. |.. |.. |.. | 1 | .. | 1 | .. | 1 | .. | .. |
|851 |1002 | P. | Do. | 8 |7526 | F1 | Do. | 3 |.. |.. |.. | 3 | 1 | 2 | 1 | 2 | 3 | 1 | 1 |
|851 |2073 | P. | Do. | 7 |7526 | F1 | Do. | 3 |.. |.. | 1 | 1 | 1 | .. | 1 | 1 | 1 | 3 | 2 |
|851 |2299 | P. | Do. | 9 |7526 | F1 | Do. | 3 |.. |.. |.. |.. | 2 | 2 | 1 | 1 | .. | .. | 2 |
|851 |3410 | P. | Do. | 9 |7526 | F1 | Do. | 3 |.. |.. |.. |.. | .. | 4 | 3 | 2 | 1 | 5 | 1 |
|851 |5567 | P. | Do. | 9 |7526 | F1 | Do. | 3 |.. |.. |.. |.. | 2 | .. | .. | .. | 3 | 3 | 5 |
|851 |6956 | P. | Do. | 8 |7526 | F1 | Do. | 3 |.. |.. |.. |.. | 3 | 3 | .. | 2 | 2 | .. | 5 |
| | | | ====================================================
| | | | Totals (99) ..... | 0 | 0 | 1 | 7 | 13 | 15 | 11 | 8 | 11 | 15 | 18 |
+----+-----+-----+---------------------------------------------+---+---+---+---+----+----+----+----+----+---------+
[A] Pure-blooded Silkie assumed heterozygous to boot.
Numerous observations have been made upon the progeny of parents
belonging to hybrid generations beyond the first. Owing to the
extreme imperfection of dominance it is rarely possible to say
with certainty from inspection whether a given bird has germ-cells
dominant or recessive, or both, with reference to booting; only
breeding enables us to make a decision. There is an exception,
however, in the case of pure extracted recessives. They are
distinguished by heavy booting and produce only booted offspring. I
propose to give, in detail, the matings of these later generations
and their progeny, the families being arranged in decreasing order of
average grade of booting (table 38).
TABLE 38.--_Distribution of boot-grades in offspring of parents
one or both of which belong to a hybrid generation beyond the
first._
B = Brahma; C = Cochin; G = Game; L = Leghorn; M = Minorca; S =
Silkie; Sp = Spanish; T = Tosa; WL = White Leghorn
+------+---+------------------------+------------------------+-------+-------------------------------------+
|Serial|Pen| Mother. | Father. |Mating.| Boot-grade in offspring. |
| No. |No.+----+-----+---------+---+----+-----+---------+---+ +--+--+--+--+--+--+--+--+--+--+--+----+
| | | No.| Gen.| Races. |Gr.| No.| Gen.| Races. |Gr.| | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10| Av.|
+------+---+----+-----+---------+---+----+-----+---------+---+-------+--+--+--+--+--+--+--+--+--+--+--+----+
| 1 |814| 354| F1 |B × T | 7|3975| F2 |B × T | 9 |R × R |..|..|..|..|..|..|..|..|..|10|15|9.6 |
| 2 |801| 181| F1 | Do. | 4|5399| F2 |M × B | 8 | Do. |..|..|..|..|..|..|..|..|..| 1| 1|9.5 |
| 3 |814| 300| F1 | Do. | 5|3975| F2 |B × T | 9 | Do. |..|..|..|..|..|..|..|..| 1| 3| 4|9.4 |
| 4 |801|4569| F2 | Do. | 6|4562| F2 | Do. | 7 | Do. |..|..|..|..|..|..|..|..| 1| 1| 2|9.3 |
| 5 |814|5523| F2 | Do. | 9|3975| F2 | Do. | 9 | Do. |..|..|..|..|..| 1|..|..| 3| 4| 9|9.1 |
| 6 |814|4560| F2 | Do. | 8|3975| F2 | Do. | 9 | Do. |..|..|..|..|..| 1| 1| 1|..| 2| 7|8.8 |
| 7 |814| 190| F1 | Do. | 2|3975| F2 | Do. | 9 | Do. |..|..|..|..|..|..| 1| 1| 1| 1| 4|8.8 |
| 8 |806|4325| F3 |M × B | 7|5257| F3 |M × B | 9 | Do. |..|..|..|..|..|..| 1| 1| 2| 2| 3|8.6 |
| 9 |806|5913| F3 | Do. | 7|5257| F3 | Do. | 9 | Do. |..|..|..|..| 1|..|..| 1| 4| 2| 3|8.3 |
| 10 |732|1235| F2 | Do. | 8|2732| F2 | Do. | 6 | Do. |..|..|..|..|..|..| 1| 2| 4| 3|..|7.9 |
| 11 |806|4052| F3 | Do. | 5|5257| F3 | Do. | 5 | Do. |..|..|..|..| 1| 1|..| 3|..| 6| 1|7.8 |
| 12 |776|1132| F2 |C × WL | 3|2732| P. |C | 8 |DR × R |..|..|..| 1| 1| 1| 1| 3| 6| 8|..|7.6 |
| 13 |801|6869| F1.5|B × F1 | 6|4562| F2 |M × B | 7 |R × R |..|..|..|..|..| 1| 1| 2| 1| 1| 1|7.4 |
| 14 |814| 186| F1 |T × B | 4|3975| F2 |B × T | 9 |DR × R |..| 2| 1| 0| 1| 3| 0| 1| 3| 6| 5|7.2 |
| 15 |814|4683| F2 | Do. | 2|3975| F2 | Do. | 9 | Do. |..|..|..|..| 3| 2| 3| 1| 1| 1| 5|7.1 |
| 16 |767|2104| F2 |WL × B | 3|3116| F1 | Do. | 9 | Do. |..|..|..| 1| 4| 1| 2| 7| 6| 1| 0|7.1 |
| 17 |801|2526| F1 | Do. | 2|5399| F2 |M × B | 8 | Do. |..|..|..|..| 1|..|..| 1|..|..| 1|7.0 |
| 18 |806|3936| F2 |M × B | 10|5257| F3 | Do. | 9 |R × R |..|..|..|..| 1|..| 2|..| 2|..| 1|7.0 |
| 19 |839|5383| F2 |L × M × B| 2|4348| F2 |L × M × B| 3 |DR × DR|..|..|..|..| 1| 1|..|..|..| 1| 1|7.0 |
| 20 |801|5515| F2 |B × T | 4|5399| F2 |M × B | 8 |DR × R |..|..|..| 1| 1| 2| 2|..| 1| 1| 3|6.9 |
| 21 |732|1003| F2 |M × B | 9|2442| F2 | Do. | 6 |R × R |..|..|..|..| 3| 7| 7| 7| 7| 5| 2|6.8 |
| 22 |839|1892| F1.5|L × M × B| 6|4348| F2 |L × M × B| 3 |R × DR |..|..|..| 2| 1|..| 1|..|..| 2| 2|6.8 |
| 23 |806|4196| F3 |M × B | 2|5257| F3 |M × B | 9 |DR × R |..|..|..| 2| 2| 2| 1|..|..| 3| 3|6.7 |
| 24 |801|2526| F1 |WL × B | 2|5399| F2 | Do. | 8 | Do. |..|..|..|..| 1|..|..| 1|..| 1| 0|6.7 |
| 25 |801|6861| F2.5|B × T | 7|4562| F2 | Do. | 7 |R × R |..|..|..|..|..| 2| 1|..|..|..| 1|6.5 |
| 26 |767| 872| F2 | Do. | 5|3116| F1 |B × T | 9 |DR × R | 1| 0| 0| 1| 4| 6| 9| 4| 4| 6| 3|6.5 |
| 27 |801|4263| F2 | Do. | 3|4562| F2 |M × B | 7 | Do. |..|..|..| 2| 3|..| 3| 1| 1| 4| 1|6.5 |
| 28 |767| 181| F1 | Do. | 4|3166| F1 |B × T | 9 | Do. |..|..| 1| 2| 6|13|11| 5|11| 8| 3|6.5 |
| 29 |814| 862| F2 | Do. | 1|3975| F2 | Do. | 9 | Do. |..|..|..| 2| 2| 5| 2| 1| 1| 3| 2|6.3 |
| 30 |801| 872| F2 | Do. | 5|5399| F2 |M × B | 8 | Do. |..|..|..| 1| 3| 8| 5| 2|..| 2| 4|6.3 |
| 31 |839|5389| F2 |M × B | 7|4348| F2 | Do. | 3 |R × DR |..|..|..| 6| 4| 1| 0| 1| 1| 1| 6|6.2 |
| 32 |801| 872| F2 |B × T | 5|4562| F2 | Do. | 7 |DR × R |..|..|..| 1| 5| 4| 3| 1| 2| 2| 2|6.1 |
| 33 |767| 190| F1 | Do. | 4|3116| F1 |B × T | 9 | Do. |..|..|..| 5| 6|11|12| 7| 4| 9|..|6.1 |
| 34 |801|1892| F1 |M × B | 3|4562| F2 |M × B | 7 | Do. |..|..| 1|..|..|..| 1|..| 2|..|..|6.0 |
| 35 |801|5515| F2 |B × T | 4|4562| F2 | Do. | 7 | Do. |..|..| 1|..|..| 1| 3|..| 1| 1|..|6.0 |
| 36 |731| 248| F1 |M × B | 4|1249| F2 |WL × B | 7 | Do. |..|..| 2| 3| 3|..| 2|..|..| 5| 2|6.0 |
| 37 |732|1228| F2 | Do. | 8|2442| F2 |M × B | 6 |R × R |..|..|..| 2| 8| 5| 6| 2| 8| 3|..|6.0 |
| 38 |732| 690| F1 | Do. | 5|2442| F2 | Do. | 6 |DR × R | 2| 0| 6| 2| 5| 5| 7|16|10| 6|..|6.0 |
| 39 |751|1919| F2 |WL × B | 8|1139| F2 |L × B | 8 |R × R |..|..|..| 5| 4| 6| 6| 1|11| 1|..|5.9 |
| 40 |732| 618| F1 |M × B | 8|2442| F2 |M × B | 6 |DR × R |..| 1| 2| 3| 2| 5| 3| 5| 9| 1|..|5.8 |
| 41 |731|1245| F2 |WL × B | 9|1249| F2 |WL × B | 7 |R × R |..|..| 1| 2| 1| 8| 2| 3| 6|..|..|5.8 |
| 42 |760| 354| F1 |B × T | 5|1270| F2 |B × T | 2 |R × DR |..|..| 1| 3| 9| 5| 8| 4| 7| 2|..|5.7 |
| 43 |701|1915| F2 |WL × B | 8|1898| F2 |WL × B | 3 | Do. |..|..|..|..| 7| 4| 3| 2| 3| 1|..|5.7 |
| 44 |801|6869| F1.5|B (M × B)| 6|5399| F2 | Do. | 8 |DR × R |..|..|..| 1|..| 1|..|..|..| 1|..|5.7 |
| 45 |801|4570| F2 |B × T | 2|4562| F2 | Do. | 7 | Do. |..|..| 1| 2| 5| 3| 1| 1| 1| 4|..|5.6 |
| 46 |814| 703| F1 | Do. | 4|3975| F2 |B × T | 9 | Do. |..|..| 3| 5| 2| 7| 5| 6| 2| 4|..|5.5 |
| 47 |732| 953| F2 |M × B | 3|2442| F2 |M × B | 6 | Do. |..| 2| 2| 3| 8| 9| 5| 3| 7| 6|..|5.5 |
| 48 |801|7528| F1 | Do. | 4|4562| F2 | Do. | 8 | Do. |..|..|..| 1| 4| 2| 1|..| 1|..| 1|5.3 |
| 49 |731|2116| F2 | Do. | 10|1249| F2 |WL × B | 7 |R × R |..| 1| 1| 1| 2| 3| 0| 2| 2| 1|..|5.2 |
| 50 |745|2115| F2 |C × T | 4|1258| F2 |B × T | 4 |DR × DR|..|..|..| 2| 1| 6| 4| 2|..|..|..|5.2 |
| 51 |801|6843| F2 |B × T | 3|4562| F2 | Do. | 8 |DR × R |..|..|..|..| 3| 2| 1|..| 1|..|..|5.1 |
| 52 |801|2831| F1 |M × B | 4|5399| F2 | Do. | 8 | Do. | 1| 1| 1| 4| 1| 7| 2| 2| 2|..| 2|5.0 |
| 53 |801|1892| F1 | Do. | 3|5399| F2 | Do. | 8 | Do. | 1| 1|..| 1| 2|..| 1| 0| 1| 1| 1|5.0 |
| 54 |801|7528| F1 | Do. | 4|4562| F2 | Do. | 8 | Do. |..|..|..| 1| 2| 1| 1|..| 1|..|..|5.0 |
| 55 |731|1755| F2 |WL × B | 6|1249| F2 |WL × B | 7 |R × R |..|..|..|..| 2| 1| 4| 1| 2|..|..|5.0 |
| 56 |745|2513| F3 |C × T | 4|1258| F2 |B × T | 4 |DR × DR|..|..|..|..| 2| 5| 2|..| 3| 1|..|5.0 |
| 57 |839|3950| F2 |M × B | 4|4348| F2 |M × B | 3 | Do. |..| 2| 3| 3| 2| 4| 1| 1| 1| 2| 2|4.95|
| 58 |754| 873| F2 |B × T | 3| 871| F2 |B × T | 2 | Do. | 1| 2| 1| 4| 1|..|..|..| 8|..|..|4.94|
| 59 |806| 599| F2 |M × B | 3|5257| F2 |M × B | 7 |DR × R |..|..| 2| 1| 2| 2| 1|..| 1|..|..|4.86|
| 60 |760| 300| F1 |B × T | 7|1270| F2 |B × T | 2 |R × DR |..|..| 2|19| 8|13| 6| 4| 5| 2| 1|4.83|
| 61 |806|4456| F2 |M × B | 1|5257| F3 |M × B | 7 |DR × R |..| 1| 1| 1| 1|..| 1|..| 1| 1|..|4.71|
+------+---+----+-----+---------+---+----+-----+---------+---+-------+--+--+--+--+--+--+--+--+--+--+--+----+
TABLE 38.--_Distribution of boot-grades in offspring of parents
one or both of which belong to a hybrid generation beyond the
first_--Continued.
B = Brahma; C = Cochin; G = Game; L = Leghorn; M = Minorca; S =
Silkie; Sp = Spanish; T = Tosa; WL = White Leghorn.
+------+---+------------------------+-----------------------+--------+--------------------------------------+
| | | Mother. | Father. | | Boot-grade in offspring. |
|Serial|Pen|----+-----+---------+---+----+-----+--------+---+ Mating.+--+--+--+--+--+--+--+--+--+--+--+-----+
| No. |No.| No.| Gen.| Races. |Gr.| No.| Gen.| Races. |Gr.| | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10| Av. |
+------+---+----+-----+---------+---+----+-----+--------+---+--------+--+--+--+--+--+--+--+--+--+--+--+-----+
| 62 |732|2407| F2 |M × B | 2|2442| F2 |M × B | 6 | DR × R |..|..| 1| 3| 4| 1| 1| 2|..| 1|..| 4.69|
| 63 |701| 894| F2 |L × B | 7|1898| F2 |L × B | 3 | R × DR | 1| 1| 2| 8| 6| 1| 2| 2| 4| 2|..| 4.62|
| 64 |760| 994| F2 |B × T | 3|1270| F2 |B × T | 3 | DR × DR|..|..|..|..| 4| 2| 1|..|..|..|..| 4.57|
| 65 |760| 981| F2 | Do. | 3|1270| F2 | Do. | 3 | Do. | 1|..| 3| 6| 1| 4| 2| 7|..|..|..| 4.54|
| 66 |701|1772| F2 |L × B | 6|1898| F2 |L × B | 3 | R × DR |..|..|..| 4| 7| 2| 2| 2|..|..|..| 4.47|
| 67 |839|3541| F1 |M × B | 6|4348| F2 |M × B | 3 | DR × DR| 4| 1| 4|..|..| 4| 2|..| 2| 1| 2| 4.30|
| 68 |842|1645| F2 | Do. | 2|4385| F2 | Do. | 4 | Do. | 3| 2| 6| 5| 6| 6| 3| 0| 2| 4| 1| 4.29|
| 69 |770|2049| F2 |L × B | 3| 926| F2 | Do. | 3 | Do. | 9| 3| 1| 6| 8| 2| 6| 6| 3| 1| 3| 4.29|
| 70 |731|2577| F1.5|L × C | 4|1249| F2 |L × B | 7 | DR × R |..|..| 2| 2| 3| 2| 2| 1|..|..|..| 4.25|
| 71 |701| 250| F1 |L × B | 3|1898| F2 | Do. | 3 | DR × DR| 3| 3| 5| 8|12|10|10| 6| 1|..|..| 4.22|
| 72 |701|1335| F2 |T × L × B| 8|1898| F2 | Do. | 3 | R × DR |..|..| 1| 9| 6| 6| 4| 1|..|..|..| 4.22|
| 73 |806|4767| F3 |M × B | 3|5257| F3 |M × B | 7 | DR × R |..|..| 1|..| 2| 1| 1|..|..|..|..| 4.20|
| 74 |740|1439| F2 |C × L | 2|1145| F2 |C × L | 3 | DR × DR| 3|..| 1| 3| 6| 4| 2|..| 2| 1|..| 4.18|
| 75 |754|3126| F2 |B × T | 4| 871| F2 |B × T | 3 | Do. |..| 2| 5|11| 7|10| 5| 0| 2| 1|..| 4.14|
| 76 |770|1645| F2 |M × B | 4| 926| F2 |M × B | 3 | Do. | 3| 2| 1| 9| 5| 5| 2| 2| 3| 1|..| 4.10|
| 77 |731| 249| F1 |L × B | 3|1249| F2 |L × B | 7 | DR × R | 7| 4| 6| 5| 7| 5| 9| 3| 6| 1|..| 4.08|
| 78 |732| 703| F1 |M × B | 3|2442| F2 |M × B | 6 | Do. | 1| 3|13|13| 8| 6| 7| 6| 3|..|..| 4.07|
| 79 |770| 720| F1 |B × L | 4| 926| F2 | Do. | 3 | DR × DR| 6| 1| 3| 9| 5| 4| 5| 1| 4| 1| 1| 4.05|
| 80 |732|2441| F2 |M × B | 0|2442| F2 | Do. | 6 | DR × R |..| 1| 6| 8| 2| 6| 0| 3| 1|..|..| 4.00|
| 81 |760|1042| F2 |B × T | 3|1270| F2 |B × T | 2 | DR × DR| 2| 3| 3| 9| 3| 5| 8| 2| 0| 1|..| 4.00|
| 82 |731| 384| F1 |L × B | 4|1249| F2 |L × B | 7 | DR × R | 2| 1| 4| 4| 3| 2| 4| 0| 1| 1|..| 3.82|
| 83 |814|4566| F2 |B × T | 2|3975| F2 |B × T | 9 | Do. | 1| 4| 2| 4| 3| 2| 1|..|..|..|..| 3.82|
| 84 |732| 599| F1 |M × B | 3|2442| F2 |M × B | 6 | Do. | 6| 5|23|10| 5| 3| 4| 5| 8| 3| 1| 3.78|
| 85 |770| 761| F1 |B × L | 3| 926| F2 | Do. | 3 | DR × DR| 7| 3| 5| 3| 7| 7| 2| 6| 1| 1|..| 3.71|
| 86 |731|1770| F2 | Do. | 7|1249| F2 |L × B | 7 | DR × R | 1|..| 8| 6| 9| 3| 2|..| 2|..|..| 3.65|
| 87 |861|5165| F2 |T × C | 10| 95| F1 |T × C | 5 | R × DR |..|..|..|10| 3| 2| 1|..|..|..|..| 3.63|
| 88 |754|3175| F2 |B × T | 2| 871| F2 |B × T | 2 | DR × DR| 1|..| 2| 1| 3| 4|..|..|..|..|..| 3.55|
| 89 |731|2102| F2 |L × B | 1|1249| F2 |L × B | 7 | DR × R | 1| 0| 4| 2| 4| 1| 1| 1|..|..|..| 3.43|
| 90 |840|1755| F2 |M × B | 6|4177| F2 | Do. | 2 | R × DR |..|..| 6| 7| 7| 3| 1|..|..|..|..| 3.42|
| 91 |701|2576| F2 |L × B | 2|1898| F2 | Do. | 3 | DR × DR| 2| 1| 1| 8|11| 2| 1|..|..|..|..| 3.35|
| 92 |842|2049| F1 | Do. | 3|4385| F2 |M × B | 4 | Do. |11| 1| 2| 8| 5| 3| 1| 0| 2| 2| 2| 3.35|
| 93 |754| 853| F2 |B × T | 1| 871| F2 |B × T | 3 | Do. | 2| 3| 4| 6| 4| 1| 6|..|..|..|..| 3.31|
| 94 |826|2652| F1 |M × B | 3|4093| F2 |M × B | 0 | Do. | 8| 2| 1| 8| 1|..|..| 1| 2| 3|..| 3.28|
| 95 |754|1052| F2 |B × T | 2| 871| F2 |B × T | 2 | Do. | 3|..| 7| 9| 9| 5| 2|..|..|..|..| 3.26|
| 96 |701| 965| F2 |T × L × B| 0|1898| F2 |L × B | 3 | Do. | 1| 4| 6|12| 8| 4| 0| 2| 0|..|..| 3.19|
| 97 |732|1833| F2 |M × B | 1|2442| F2 |M × B | 6 | DR × R | 1| 1| 7| 6| 6| 4| 1|..|..|..|..| 3.19|
| 98 |732| 631| F1 | Do. | 3|2442| F2 | Do. | 6 | Do. | 3| 4|10|16|12| 4| 1| 2|..|..|..| 3.08|
| 99 |754| 862| F2 |B × T | 1| 871| F2 |B × T | 2 | DR × DR| 1| 5|10|17|10| 4| 1|..|..|..|..| 2.96|
| 100 |837|5641| F2 |T × L × B| 0|4288| F3 |L × B | 2 | Do. | 1| 2| 2| 3|..| 2|..| 1|..|..|..| 2.91|
| 101 |840|3841| F2 |L × B | 0|4177| F2 | Do. | 2 | D × DR | 3| 3| 2| 6| 4| 2|..|..|..|..|..| 2.86|
| 102 |701| 721| F1 | Do. | 2|1898| F2 | Do. | 3 | DR × DR| 2| 4| 3| 8| 3| 2| 2|..|..|..|..| 2.83|
| 103 |839|3949| F2 | Do. | 4|4348| F2 | Do. | 3 | Do. | 1| 2|..|..| 1| 1| 1|..|..|..|..| 2.83|
| 104 |840| 732| F1 | Do. | 3|4177| F2 | Do. | 2 | Do. | 7| 6| 9| 8| 7| 2| 1| 2|..| 1|..| 2.67|
| 105 |840| 249| F1 | Do. | 3|4177| F2 | Do. | 2 | Do. | 7| 3| 5| 6| 2| 9|..|..|..|..|..| 2.62|
| 106 |840|3916| F1.5| Do. | 2|4177| F2 | Do. | 2 | Do. | 5| 1| 4| 2| 2| 2| 1|..|..|..|..| 2.29|
| 107 |842|4945| F2 |M,L × B | 1|4385| F2 |M × B | 4 | Do. | 9| 3| 6| 5| 1| 2| 4|..|..|..|..| 2.27|
| 108 |731|2595| F2 |L × B | 1|1249| F2 |L × B | 7 | D × R | 6| 6| 7| 1|..|..|..|..|..|..|..| 2.15|
| 109 |840|5169| F2 | Do. | 3|4177| F2 | Do. | 2 | DR × DR| 6| 2| 5| 5| 2| 2|..|..|..|..|..| 2.05|
| 110 |837|5667| F3 | Do. | 2|4288| F3 | Do. | 2 | Do. | 2| 1| 2|..| 1| 1|..|..|..|..|..| 2.00|
| 111 |749|1355| F2 |G × C | 2|1854| F2 |G(C × L)| 0 | DR × D |..| 2| 5| 1|..|..|..|..|..|..|..| 1.87|
| 112 |824|3901| F2 |M × S | 1|5095| F2 |M × S | 1 | DR × DR|17| 3| 2| 3|..| 2|..|..| 1| 2|..| 1.73|
| 113 |751|1254| F2 |L × B | 0|1139| F2 |L × B | 8 | D × R |17| 5| 5| 4| 3| 3|..| 1|..|..|..| 1.63|
| 114 |749| 816| F1 | Do. | 2|1854| F2 |G(C × L)| 0 | DR × D | 6| 7| 3| 5| 1|..|..|..|..|..|..| 1.45|
| 115 |749| 929| F2 |G × C | 0|1854| F2 | Do. | 0 | D × D | 8| 3| 1|..|..|..| 1|..|..| 1|..| 1.43|
| 116 |749| 819| F1 |L × B | 1|1854| F1.5|G(C × L)| 0 | DR × D | 9| 3| 5| 3|..|..|..|..|..|..|..| 1.10|
| 117 |804|5099| F2 |S × Sp | 0|3823| F1 |S × Sp | 0 | D × D | 2|..|..| 1|..|..|..|..|..|..|..| 1.00|
| 118 |804|6043| F2 | Do. | 1|3823| F1 | Do. | 0 | Do. |..| 1|..|..|..|..|..|..|..|..|..| 1.00|
| 119 |817|5730| F1 |L × Sp | 0|3900| F2 | Do. | 1 | D × DR | 3| 3| 1|..|..|..|..|..|..|..|..| 0.71|
| 120 |817|4696| F1 | Do. | 0|3900| F2 | Do. | 1 | Do. | 9| 7| 3|..|..|..|..|..|..|..|..| 0.68|
| 121 |817|6046| F2 |S × M | 0|3900| F2 | Do. | 1 | Do. |10|..| 3|..|..|..|..|..|..|..|..| 0.46|
| 122 |817|6833| F1.5|L(G × S) | 0|3900| F2 | Do. | 1 | Do. | 6| 2| 1|..|..|..|..|..|..|..|..| 0.44|
| 123 |817|5062| F1 |L(Sp) | 0|3900| F2 | Do. | 1 | Do. |18| 7| 2|..|..|..|..|..|..|..|..| 0.41|
| 124 |817|5069| F1 | Do. | 0|3900| F2 | Do. | 1 | Do. |21| 7| 2|..|..|..|..|..|..|..|..| 0.37|
| 125 |817|6406| F1 | Do. | 0|3900| F2 | Do. | 1 | Do. |25| 8| 2|..|..|..|..|..|..|..|..| 0.34|
| 126 |817|7047| F1 | Do. | 0|3900| F2 | Do. | 1 | Do. | 4| 2|..|..|..|..|..|..|..|..|..| 0.33|
| 127 |749|2651| F2 |G × C | 0|1854| F2 |G(C × L)| 0 | D × D | 2| 1|..|..|..|..|..|..|..|..|..| 0.33|
| 128 |824|4714| F2 |S × Sp | 0|5095| F2 |M × S | 1 | Do. |26| 6| 1| 1|..|..|..|..|..|..|..| 0.32|
| 129 |817|4690| F1 | Do. | 0|3900| F2 |S × Sp | 1 | D × DR |21| 6| 1|..|..|..|..|..|..|..|..| 0.29|
| 130 |824|7439| F2 | Do. | 0|5095| F2 |M × S | 1 | D × D |11| 4|..|..|..|..|..|..|..|..|..| 0.27|
| 131 |804|4715| F2 | Do. | 0|3823| F1 |S × Sp | 0 | DR × DR|18| 2| 1|..|..|..|..|..|..|..|..| 0.19|
| 132 |804|3898| F2 |S × M | 0|3823| F1 | Do. | 0 | D × DR |19|..|..|..|..|..|..|..|..|..|..| 0.00|
| 133 |804|3902| F2 | Do. | 0|3823| F1 | Do. | 0 | Do. |33|..|..|..|..|..|..|..|..|..|..| 0.00|
| 134 |804|4657| F2 | Do. | 0|3823| F1 | Do. | 0 | Do. | 8|..|..|..|..|..|..|..|..|..|..| 0.00|
| 135 |804|4716| F2 | Do. | 0|3823| F1 | Do. | 0 | Do. |19|..|..|..|..|..|..|..|..|..|..| 0.00|
| 136 |804|5431| F2 | Do. | 0|3823| F1 | Do. | 0 | Do. |16|..|..|..|..|..|..|..|..|..|..| 0.00|
+------+---+----+-----+---------+---+----+-----+--------+---+--------+--+--+--+--+--+--+--+--+--+--+--+-----+
In table 38 I have given in the section lying between that headed
"Father" and that headed "Offspring" the "Matings." This column
differs from the others of the table in not being, in general,
based upon observation, but upon a sometimes complicated judgment.
Of course, all of the F1 generation, where this generation occurs,
may be taken as of DR composition; but the decision as to whether
a given individual of F2 is a DR, an extracted dominant, or an
extracted recessive is not always easy, because of the manifestation
of imperfect dominance. But the assignments are by no means
arbitrary. Taking the Brahma crosses, which are by far the most
numerous, we see, from tables 31, B and 33, that those F2 individuals
that have a boot of grade 6 or higher are almost certainly extracted
recessives (which are equivalent to pure-bred Dark Brahmas). Those
with a grade of 3 or even 4 and lower to 2 or even 1 are probably
heterozygotes, while those with grade 0 and some of those with grade
1 are extracted dominants. In cases of doubt the distribution of
grades in the offspring will give the deciding vote. In case the
individual has been used as a parent in more than one mating the
results in all the matings are taken into account, for the germinal
constitution of an individual must be regarded as fixed at all times
and in all matings. The assignment under "Matings" has, then, been
made by the application of the above rules.
In tables 39 to 43 there are grouped together the progeny from
matings of the same sort, selecting from table 38 the crosses into
which the Dark Brahma enters as the booted parent.
TABLE 39.--_RR × RR crosses from table 38._
+-------+-----------------------------------------------------+----------------------+
| | Boot-grade in offspring. | Parental grades. |
|Serial +--+---+---+---+---+----+----+---+----+----+----+-----+-------+-----+--------+
| No. | 0| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Avge.|Female.|Male.|Average.|
+-------+--+---+---+---+---+----+----+---+----+----+----+-----+-------+-----+--------+
| 1 |..| ..| ..| ..| ..| ..| ..| ..| ..| 10| 15| 9.6| 7 | 9 | 8.0 |
| 2 |..| ..| ..| ..| ..| ..| ..| ..| ..| 1| 1| 9.5| 4 | 8 | 6.0 |
| 3 |..| ..| ..| ..| ..| ..| ..| ..| 1| 3| 4| 9.4| 5 | 9 | 7.0 |
| 4 |..| ..| ..| ..| ..| ..| ..| ..| 1| 1| 2| 9.3| 6 | 7 | 6.5 |
| 5 |..| ..| ..| ..| ..| 1| ..| ..| 3| 4| 9| 9.1| 9 | 9 | 9.0 |
| 6 |..| ..| ..| ..| ..| 1| 1| 1| ..| 2| 7| 8.8| 8 | 9 | 8.5 |
| 7 |..| ..| ..| ..| ..| ..| 1| 1| 1| 1| 4| 8.8| 2 | 9 | 5.5 |
| 8 |..| ..| ..| ..| ..| ..| 1| 1| 2| 2| 3| 8.6| 7 | 5 | 6.0 |
| 9 |..| ..| ..| ..| 1| ..| ..| 1| 4| 2| 3| 8.3| 7 | 5 | 6.0 |
| 10 |..| ..| ..| ..| ..| ..| 1| 2| 4| 3| ..| 7.9| 8 | 6 | 7.0 |
| 11 |..| ..| ..| ..| 1| 1| ..| 3| ..| 6| 1| 7.8| 5 | 5 | 5.0 |
| 13 |..| ..| ..| ..| ..| 1| 1| 2| 1| 1| 1| 7.4| 6 | 7 | 6.5 |
| 18 |..| ..| ..| ..| 1| ..| 2| ..| 2| ..| 1| 7.0| 10 | 9 | 9.5 |
| 21 |..| ..| ..| ..| 3| 7| 7| 7| 7| 5| 2| 6.8| 9 | 6 | 7.5 |
| 25 |..| ..| ..| ..| ..| 2| 1| ..| ..| ..| 1| 6.5| 7 | 7 | 7.0 |
| 37 |..| ..| ..| 2| 8| 5| 6| 2| 8| 3| ..| 6.0| 8 | 6 | 7.0 |
| 39 |..| ..| ..| 5| 4| 6| 6| 1| 11| 1| ..| 5.9| 8 | 8 | 8.0 |
| 41 |..| ..| 1| 2| 1| 8| 2| 3| 6| ..| ..| 5.8| 9 | 7 | 8.0 |
| 49 |..| 1| 1| 1| 2| 3| ..| 2| 2| 1| ..| 5.2| 10 | 7 | 8.5 |
| 55 |..| ..| ..| 2| 1| 4| 1| 2| ..| ..| ..| 5.0| 6 | 7 | 6.5 |
| +--+---+---+---+---+----+----+---+----+----+----+-----+ | | |
|Totals | | | | | | | | | | | | | | | |
| (287)|..| 1| 2| 12| 22| 39| 30| 28| 53| 46| 54| 7.25| | | |
|Per | | | | | | | | | | | | | | | |
| cent.|..|0.3|0.7|4.2|7.7|13.6|10.5|9.8|18.5|16.0|18.8| ...| | | |
+-------+--+---+---+---+---+----+----+---+----+----+----+-----+-------+-----+--------+
TABLE 40.--_DR × RR crosses from table 38._
+-----------+-------------------------------------------------------------+
| | Boot-grade in offspring. |
| Serial +---+---+---+----+----+-----+----+----+----+----+----+--------+
| No. | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |Average.|
+-----------+---+---+---+----+----+-----+----+----+----+----+----+--------+
| 14 | ..| 2| 1| ..| 1| 3 | ..| 1 | 3 | 6 | 5 | 7.2 |
| 15 | ..| ..| ..| ..| 3| 2 | 3| 1 | 1 | 1 | 5 | 7.1 |
| 16 | ..| ..| ..| 1| 4| 1 | 2| 7 | 6 | 1 | .. | 7.1 |
| 17 | ..| ..| ..| ..| 1| .. | ..| 1 | .. | .. | 1 | 7.0 |
| 20 | ..| ..| ..| 1| 1| 2 | 2| .. | 1 | 1 | 3 | 6.9 |
| 22 | ..| ..| ..| 2| 1| .. | 1| .. | .. | 2 | 2 | 6.8 |
| 23 | ..| ..| ..| 2| 2| 2 | 1| .. | .. | 3 | 3 | 6.7 |
| 24 | ..| ..| ..| ..| 1| .. | ..| 1 | .. | 1 | .. | 6.7 |
| 26 | 1| ..| ..| 1| 4| 6 | 9| 4 | 4 | 6 | 3 | 6.5 |
| 27 | ..| ..| ..| 2| 3| .. | 3| 1 | 1 | 4 | 1 | 6.5 |
| 28 | ..| ..| 1| 2| 6| 13 | 11| 5 | 11 | 8 | 3 | 6.3 |
| 29 | ..| ..| ..| 2| 2| 5 | 2| 1 | 1 | 3 | 2 | 6.3 |
| 30 | ..| ..| ..| 1| 3| 8 | 5| 2 | .. | 2 | 4 | 6.3 |
| 31 | ..| ..| ..| 6| 4| 1 | ..| 1 | 1 | 1 | 6 | 6.2 |
| 32 | ..| ..| ..| 1| 5| 4 | 3| 1 | 2 | 2 | 2 | 6.1 |
| 33 | ..| ..| ..| 5| 6| 11 | 12| 7 | 4 | 9 | .. | 6.1 |
| 34 | ..| ..| 1| ..| ..| .. | 1| .. | 2 | .. | .. | 6.0 |
| 35 | ..| ..| 1| ..| ..| 1 | 3| .. | 1 | 1 | .. | 6.0 |
| 36 | ..| ..| 2| 3| 3| .. | 2| .. | .. | 5 | 2 | 6.0 |
| 40 | ..| 1| 2| 3| 2| 5 | 3| 5 | 9 | 1 | .. | 5.8 |
| 42 | ..| ..| 1| 3| 9| 5 | 8| 4 | 7 | 2 | .. | 5.7 |
| 43 | ..| ..| ..| ..| 7| 4 | 3| 2 | 3 | 1 | .. | 5.7 |
| 44 | ..| ..| ..| 1| ..| 1 | ..| .. | .. | 1 | .. | 5.7 |
| 45 | ..| ..| 1| 2| 5| 3 | 1| 1 | 1 | 4 | .. | 5.6 |
| 46 | ..| ..| 3| 5| 2| 7 | 5| 6 | 2 | 4 | .. | 5.5 |
| 47 | ..| 2| 2| 3| 8| 9 | 5| 3 | 7 | 6 | .. | 5.5 |
| 48 | ..| ..| ..| 1| 4| 2 | 1| .. | 1 | .. | 1 | 5.3 |
| 51 | ..| ..| ..| ..| 3| 2 | 1| .. | 1 | .. | .. | 5.1 |
| 52 | 1| 1| 1| 4| 1| 7 | 2| 2 | 2 | .. | 2 | 5.0 |
| 53 | 1| 1| ..| 1| 2| .. | 1| .. | 1 | 1 | 1 | 5.0 |
| 54 | ..| ..| ..| 1| 2| 1 | 1| .. | 1 | .. | .. | 5.0 |
| 59 | ..| ..| 2| 1| 2| 2 | 1| .. | 1 | .. | .. | 4.9 |
| 60 | ..| ..| 2| 19| 8| 13 | 6| 4 | 5 | 2 | 1 | 4.8 |
| 61 | ..| 1| 1| 1| 1| .. | 1| .. | 1 | 1 | .. | 4.8 |
| 62 | ..| ..| 1| 3| 4| 1 | 1| 2 | .. | 1 | .. | 4.7 |
| 63 | 1| 1| 2| 8| 6| 1 | 2| 2 | 4 | 2 | .. | 4.6 |
| 66 | ..| ..| ..| 4| 7| 2 | 2| 2 | .. | .. | .. | 4.5 |
| 70 | ..| ..| 2| 2| 3| 2 | 2| 1 | .. | .. | .. | 4.3 |
| 72 | ..| ..| 1| 9| 6| 6 | 4| 1 | .. | .. | .. | 4.2 |
| 73 | ..| ..| 1| ..| 2| 1 | 1| .. | .. | .. | .. | 4.2 |
| 77 | 7| 4| 6| 5| 7| 5 | 9| 3 | 6 | 1 | .. | 4.1 |
| 78 | 1| 3| 13| 13| 8| 6 | 7| 6 | 3 | .. | .. | 4.1 |
| 80 | ..| 1| 6| 8| 2| 6 | ..| 3 | 1 | .. | .. | 4.0 |
| 82 | 2| 1| 4| 4| 3| 2 | 4| .. | 1 | 1 | .. | 3.8 |
| 83 | 1| 4| 2| 4| 3| 2 | 1| .. | .. | .. | .. | 3.8 |
| 84 | 6| 5| 23| 10| 5| 3 | 4| 5 | 8 | 3 | 1 | 3.8 |
| 86 | 1| ..| 8| 6| 9| 3 | 2| .. | 2 | .. | .. | 3.7 |
| 89 | 1| ..| 4| 2| 4| 1 | 1| 1 | .. | .. | .. | 3.4 |
| 90 | ..| ..| 6| 7| 7| 3 | 1| .. | .. | .. | .. | 3.4 |
| 97 | 1| 1| 7| 6| 6| 4 | 1| .. | .. | .. | .. | 3.2 |
| 98 | 3| 4| 10| 16| 12| 4 | 1| 2 | .. | .. | .. | 3.1 |
| +---+---+---+----+----+-----+----+----+----+----+----+--------+
|Total(1199)| 27| 32|117| 181| 200| 172 | 142| 88 |105 | 87 | 48 | 5.04 |
|Per cent. |2.3|2.7|9.8|15.1|16.7|14.3 |11.9|7.3 |8.8 |7.2 |4.0 | ... |
+-----------+---+---+---+----+----+-----+----+----+----+----+----+--------+
TABLE 41.--_DR × DD crosses._
+---------+------------------------------------------------+
| | Boot-grade in offspring. |
| Serial +------+------+------+------+-----+-----+--------+
| No. | 0 | 1 | 2 | 3 | 4 | 5 |Average.|
+---------+------+------+------+------+-----+-----+--------+
| 101 | 3 | 3 | 2 | 6 | 4 | 2 | 2.9 |
| 113 | 6 | 7 | 3 | 5 | 1 | ... | 1.5 |
| 116 | 9 | 3 | 5 | 3 | ... | ... | 1.1 |
| +------+------+------+------+-----+-----+--------+
|Total(62)| 18 | 13 | 10 | 14 | 5 | 2 | 1.69 |
|Per cent.| 29.5 | 21.3 | 16.4 | 23.0 | 8.2 | 1.6 | ... |
+---------+------+------+------+------+-----+-----+--------+
TABLE 42.--_DR x DR crosses._
+-----------+---------------------------------------------------------+
| | Boot-grade in offspring. |
|Serial No. +----+---+----+----+----+----+---+---+---+---+---+--------+
| | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10|Average.|
+-----------+----+---+----+----+----+----+---+---+---+---+---+--------+
| 19 | ..| ..| ..| ..| 1| 1| ..| ..| ..| 1| ..| 7.0 |
| 54 | ..| ..| ..| 2| 1| 6| 4| 2| ..| ..| ..| 5.2 |
| 56 | ..| ..| ..| 2| 5| 2| ..| 3| 1| ..| ..| 5.0 |
| 57 | ..| 2| 3| 3| 2| 4| 1| 1| 1| 2| 2| 5.0 |
| 58 | 1| 2| 1| 4| 1| ..| ..| ..| 8| ..| ..| 4.9 |
| 59 | ..| ..| 2| 1| 2| 2| 1| ..| 1| ..| ..| 4.9 |
| 64 | ..| ..| ..| ..| 4| 2| 1| ..| ..| ..| ..| 4.6 |
| 65 | 1| ..| 3| 6| 1| 4| 2| 7| ..| ..| ..| 4.5 |
| 67 | 4| 1| 4| ..| ..| 4| 2| ..| 2| 1| 2| 4.3 |
| 68 | 3| 2| 6| 5| 6| 6| 3| ..| 2| 4| 1| 4.3 |
| 69 | 9| 3| 1| 6| 8| 2| 6| 6| 3| 1| 3| 4.3 |
| 71 | ..| ..| 2| 2| 3| 2| 2| 1| ..| ..| ..| 4.3 |
| 75 | ..| 2| 5| 11| 7| 10| 5| ..| 2| 1| ..| 4.1 |
| 76 | 3| 2| 1| 9| 5| 5| 2| 2| 3| 1| ..| 4.1 |
| 79 | 6| 1| 3| 9| 5| 4| 5| 1| 4| 1| 1| 4.1 |
| 81 | 2| 3| 3| 9| 3| 5| 8| 2| ..| 1| ..| 4.0 |
| 85 | 7| 3| 5| 3| 7| 7| 2| 6| 1| 1| ..| 3.7 |
| 88 | 1| ..| 2| 1| 3| 4| ..| ..| ..| ..| ..| 3.6 |
| 91 | 2| 1| 1| 8| 11| 2| 1| ..| ..| ..| ..| 3.4 |
| 92 | 11| 1| 2| 8| 5| 3| 1| ..| 2| 2| 2| 3.4 |
| 93 | 2| 3| 4| 6| 4| 1| 6| ..| ..| ..| ..| 3.3 |
| 94 | 8| 2| 1| 8| 1| ..| ..| 1| 2| 3| ..| 3.3 |
| 95 | 3| ..| 7| 9| 5| 2| ..| ..| ..| ..| ..| 3.3 |
| 96 | 1| 4| 6| 12| 8| 4| ..| 2| ..| ..| ..| 3.2 |
| 99 | 1| 5| 10| 17| 10| 4| 1| ..| ..| ..| ..| 3.0 |
| 100 | 1| 2| 2| 3| ..| 2| ..| 1| ..| ..| ..| 2.9 |
| 102 | 2| 4| 3| 8| 3| 2| 2| ..| ..| ..| ..| 2.8 |
| 103 | 1| 2| ..| ..| 1| 1| 1| ..| ..| ..| ..| 2.8 |
| 104 | 7| 6| 9| 8| 7| 2| 1| 2| ..| 1| ..| 2.7 |
| 105 | 7| 3| 5| 6| 2| 9| ..| ..| ..| ..| ..| 2.6 |
| 106 | 5| 1| 4| 2| 2| 2| 1| ..| ..| ..| ..| 2.3 |
| 107 | 9| 3| 6| 5| 1| 2| 4| ..| ..| ..| ..| 2.3 |
| 109 | 6| 2| 5| 5| 2| 2| ..| ..| ..| ..| ..| 2.1 |
| 110 | 2| 1| 2| ..| 1| 1| ..| ..| ..| ..| ..| 2.0 |
| +----+---+----+----+----+----+---+---+---+---+---+--------+
|Total (851)| 105| 61| 108| 178| 127| 109| 62| 37| 32| 20| 12| 3.59 |
|Per cent. |12.3|7.2|12.7|20.9|14.9|12.8|7.3|4.4|3.8|2.3|1.4| ... |
+-----------+----+---+----+----+----+----+---+---+---+---+---+--------+
TABLE 43.--_DD x DD (Silkie crosses)._
+-----------+----------------------------+
| | Boot-grade in offspring. |
|Serial No. +----+----+----+----+--------+
| | 0 | 1 | 2 | 3 |Average.|
+-----------+----+----+----+----+--------+
| 117 | 2| ..| ..| 1| 1.00 |
| 118 | ..| 1| ..| ..| 1.00 |
| 128 | 26| 6| 1| 1| 0.32 |
| 130 | 11| 4| ..| ..| 0.27 |
| 131 | 18| 2| 1| ..| 0.19 |
| 132 | 19| ..| ..| ..| 0.0 |
| 133 | 33| ..| ..| ..| 0.0 |
| 134 | 8| ..| ..| ..| 0.0 |
| 135 | 19| ..| ..| ..| 0.0 |
| 136 | 16| ..| ..| ..| 0.0 |
| +----+----+----+----+--------+
|Total (169)| 152| 13| 2| 2| 0.14 |
|Per cent. |89.9| 7.7| 1.2| 1.2| .... |
+-----------+----+----+----+----+--------+
The significance of the data given in tables 39 to 43 is best brought
out by summarizing them. Especially instructive is a comparison of
the pure-bred with the hybrids. Since the data are most complete in
the case of the Brahma crosses, these will be considered in most
detail. So far as they go, the results with the Cochins and Silkies
are entirely confirmatory.
Table 44 shows clearly, first, that there are families of two booted
parents that never fail to produce booted offspring. There is,
however, even in pure-bred booted races, a marked variability in the
grade of booting, extending from 3 (or 4) to 10. The significance of
this variability must be left for future investigations. There is in
the least boot, as it were, an extension of the field of activity of
the feather-inhibiting factor that is always present on the hinder
aspect of the shank, so that it interferes with the development of
feathers on the inner face of the shank also.
In the first hybrid generation all somatic cells are hybrid. The
feather inhibitor is present in the skin of the shank, but its
strength is diluted by the presence in the same cells of a protoplasm
devoid of the inhibiting property. Consequently, the prevailing
grade of the boot falls from 6 (or 10) to 3. Despite the dilution,
inhibition is complete in about 8 per cent of the offspring (grade
0); in about 10 per cent of the offspring the inhibiting factor is
so weak that the boot develops as in the pure-blooded Brahma. When,
as a result of inbreeding F1's, the feather-inhibiting factor is
eliminated from certain offspring, and such full-feathered birds are
bred together, we find a return of the mode to high numbers, such as
8 to 10 (but also 5). There is no doubt of segregation.
TABLE 44.--_Brahma crosses. (All entries are percentages.)_
+---------------+------+----------------------------------------------------------------------------+
| | | Boot-grade in offspring. |
| Percentage. | From +------+------+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
| |table.| | | | | | | | | | | |Average |
| | | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | grade. |
+---------------+------+------+------+-----+-----+-----+-----+-----+-----+-----+-----+-----+--------+
|Pure blood | 31, B| .... | .... | ....| 3.3| 3.3| 6.6| 24.6| 4.9| 9.8| 14.8| 32.8| 7.62 |
|F1 (D × R) | 32 | 7.9 | 13.8 | 16.8| 31.0| 17.5| 7.8| 3.4| 1.1| 0.7| ....| ....| 2.84 |
|Extracted R × R| 39 | .... | 0.3 | 0.7| 4.2| 7.7| 13.6| 10.5| 9.8| 18.5| 16.0| 18.8| 7.25 |
|DR × RR | 40 | 2.3 | 2.7 | 9.8| 15.1| 16.7| 14.3| 11.9| 7.3| 8.8| 7.2| 4.0| 5.04 |
| | |\---------------\/---------------/ \--------------\/-------------/| |
| | | 50 p. ct. DR. 50 p. ct. RR. | |
|DR × DR | 42 | 12.3 | 7.2 | 12.7| 20.9| 14.9| 12.8| 7.3| 4.4| 3.8| 2.3| 1.4| 3.59 |
| | |\------\/------/ \------\/------/ \--------------\/-------------/| |
| | | 25 p. ct. DD. 50 p. ct. DR. 25 p. ct. RR. | |
|DR × DD | 41 | 29.5 | 21.3 | 16.4| 23.0| 8.2| 1.6| ....| ....| ....| ....| ....| 1.69 |
| | |\-----\/----/|\----------\/---------/| | | | | | |
| | |50 p. ct. DD.| 50 p. ct. DR. | | | | | | |
+---------------+------+-------------+-----------------------+-----+-----+-----+-----+-----+--------+
If a heterozygous bird be mated to a recessive the variability
of the offspring is much increased, owing to the occurrence in
the progeny of both DR and RR individuals (table 40). The
offspring do not, to be sure, fall into two distinct and well-defined
types, as in typical Mendelian cases; but one part of the range of
variation agrees fairly with that of pure RR's, _i. e._, Brahmas,
and the remainder with that of heterozygotes. And if we make the
division in the middle of the middle class, viz, 5, we shall find
a close approximation to that equality of extracted recessives and
heterozygotes that the segregation theory calls for (table 44).
If, again, two heterozygous birds be mated, the variability is still
greater and the proportion of clean-footed offspring rises to 12 per
cent. These, together with some of the extremely slightly booted
offspring, represent the extracted dominants. The whole range now
falls into three regions divided by the middle of grades 2 and 5.
These regions correspond to the DD's, the DR's, and the RR's
of typical cases of segregation, and their relative proportions are
approximately as 25: 50: 25.
Finally, if a heterozygote be mated to an extracted dominant the
proportion of clean-footed offspring rises to about 30 per cent and
the whole range of variation falls readily into two parts, the one
comprising grades 0 and 1, the other grades 2 and above. The first
includes the DD offspring; the second, the DR's; and their
frequency is equal. One will not fail to note that we are not here
dealing with a case of blending simply, and the inheritance of the
blend; such a view is negatived by the fact of the much greater
variability of DR × DR cross over the simple D × R cross of
the first generation. One may safely conclude, then, that, despite
the apparent blending of booting characters in the first generation
of hybrids, true segregation takes place. But this is always to be
seen through the veil of imperfect dominance.
A casual examination of table 38 would seem to show a correlation
between the grade of booting of the parents and that of the average
of their progeny. Thus, on the whole, the parental grades run high
in the upper part of the table and run low in the lower part.
This relation would thus seem to confirm Castle's conclusion for
polydactylism in guinea-pigs that there is an inheritance of the
degree of a character. One consequence of such an inheritance would
be that it would be possible in a few generations to increase or
diminish the grade of a character and fix any required grade in the
germ-plasm. A more careful consideration of the facts of the case
shows that this relation has another interpretation. The grade of
boot of the different parents varies largely because their gametic
constitution is diverse. As table 39 shows, the parents of the upper
part of table 38 are chiefly extracted recessives, and consequently
their booting and that of their offspring are characterized by high
grades. On the other hand, the parents of the lower part of the table
are heterozygous or extracted dominants and, consequently, their
grades and also those of their offspring average low. On account of
the lack of homogeneity of the families in table 38, one can draw
from it no proper conclusions as to relation between parental and
filial grades. On the other hand, from a homogeneous table, like
table 39, we can hope to reach a conclusion as to the existence of
such a relation. I have calculated, in the usual biometric fashion,
the coefficient of correlation between average parental and filial
grades, and found it to be -0.17 ± 0.13. This can only be interpreted
to mean that in a homogeneous assemblage of families there is no
correlation between the grade of booting of parents and offspring.
CHAPTER VII.
NOSTRIL-FORM.
In my 1906 report I described in detail the form of the nostril in
poultry. Usually it is closed down to a narrow slit, but in some
races, as, _e. g._, the Polish and Houdans, the closing flap of
skin fails to develop and the nostril remains wide open. This is
apparently an embryonic condition. Thus in Keibel and Abraham's
(1900) Normaltafeln of the fowl it is stated that the outer nasal
opening, which is at first wide open, becomes closed with epithelium
at about the middle of the sixth day of development. The Polish and
Houdan fowl thus retain in the outer nasal opening an embryonic
condition. The question is: How does this embryonic, open condition
of the nostril behave in heredity with reference to the more advanced
narrow-slit condition?
The wide-nostriled races used were both the Polish and the Houdan.
The condition of the external nares is much the same in the two,
but is slightly more exaggerated in the Houdans than in the Polish.
The open nostril is often associated with a fold across the
culmen, apparently due to the upturning of the anterior end of the
premaxillary process of the nasal bone. Breeders of Houdans have
sought to exaggerate the height of the fold. In both races there
is great variability in the degree of "openness" of the nostril,
and to indicate this I have adopted a scale of 10 grades (running
from 1, the narrowest, to 10, the widest). To get some idea of this
variability let us consider the grade of nostril in some families of
pure Houdans.
TABLE 45.--_Variability (expressed in decimal grades) of the
degree of "openness" of the nostrils in families of "pure-bred"
Houdans._
+------+---+-----------+-----------+--------------------------------------+
| | | Mother. | Father. | Grade of openness in offspring. |
|Serial|Pen| | | |
| No. |No.+----+------+----+------+--+--+--+--+---+---+---+----+----+----+
| | | No.|Grade.| No.|Grade.| 1| 2| 3| 4| 5 | 6 | 7 | 8 | 9 | 10 |
+------+---+----+------+----+------+--+--+--+--+---+---+---+----+----+----+
| 1 |727|2457| 9 | 831| 10 |..|..|..|..| ..| ..| ..| .. | 5 | 4 |
| 2 |727|2459| 10 | 831| 10 |..|..| 1|..| ..| ..| 1| 3 | 7 | 3 |
| 3 |727|2494| 9 | 831| 10 |..|..|..|..| ..| ..| ..| .. | 1 | 4 |
| 4 |727|3105| 9 | 831| 10 |..| 1|..| 1| 2| 1| ..| 5 | 7 | 3 |
| 5 |727|3106| 9 | 831| 10 |..|..|..|..| ..| ..| ..| .. | 2 | 1 |
| 6 |803|2457| 8 |7522| 9 |..| 1| 1|..| ..| 2| 4| 7 | 10 | 3 |
| 7 |803|2459| 10 |7522| 9 |..|..|..|..| ..| ..| 1| 6 | 4 | 2 |
| 8 |803|3105| 9 |7522| 9 | 1|..|..|..| 4| 2| 2| 7 | 3 | 7 |
| | +--+--+--+--+---+---+---+----+----+----+
| | Totals (119) | 1| 2| 2| 1| 6| 5| 8| 28 | 39 | 27 |
| | |\----\/---/| | | | | | |
| | Percentages | 5.3 |5.3|4.4|7.1|24.8|34.5|23.9|
+------+---------------------------+-----------+---+---+---+----+----+----+
Table 45 shows that the prevailing grade in the offspring of pure
Houdans is 9; that grades 8 and 10 are also extremely common; and
that lower grades, even down to 1, may occur, but these are much less
common.
We have next to consider the grade-distribution of the offspring of
the narrow mated with the wide nostril.
TABLE 46.--_Distribution of the frequency of the different grades
of "openness" of nostril when one parent has the open nostril and
the other the closed._
+--------+---+--------------------------+--------------------------+-----------------------------------------+
| | | Mother. | Father. | Grade of openness in offspring. |
| Serial |Pen+---+-----+------------+---+----+-----+-----------+---+----+----+----+----+---+---+---+---+--+--+
| No. |No.|No.| Gen.| Races. |Gr.| No.| Gen.| Races. |Gr.| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9|10|
+--------+---+---+-----+------------+---+----+-----+-----------+---+----+----+----+----+---+---+---+---+--+--+
| 9 |727|121| P. |Dk. Brahma. | 1 | 831| P. |Houdan | 10| 9 | 11 | 6 | 6 | 2| 3| 1| 1|..|..|
| 10 |735|142| P. |Mediterran. | 1 | 30| P. |Polish | 8| 4 | 1 | .. | .. | ..| ..| ..| ..|..|..|
| 11 |735|177| P. | Do. | 1 | 30| P. | Do. | 8| .. | 4 | 2 | 1 | ..| ..| ..| ..|..|..|
| 12 |735|198| P. | Do. | 1 | 30| P. | Do. | 8| .. | 3 | 1 | .. | ..| 1| ..| ..|..|..|
| | | | | | +----+----+----+----+---+---+---+---+--+--+
| | | | | Totals (56) | ..| 13 | 19 | 9 | 7 | 2| 4| 1| 1|..|..|
| | | | | Percentages | ..|23.2|34.0|16.1|12.5|3.6|7.1|1.8|1.8|..|..|
| | | | | | +====+====+====+====+===+===+===+===+==+==+
|[A]12_a_|813|912| F2 |Houd × Legh.| 2 |3904| F3 |Houd × Legh.| 7| 3 | 10 | 3 | 1 | 1| ..| ..| ..|..|..|
+--------+---+---+-----+------------+---+----+----+------------+---+----+----+----+----+---+---+---+---+--+--+
[A]: Extracted D × R.
Table 46 gives us a picture of the nature of the dominance in this
case. At first sight the narrow nostril, grades 1 and 2, including
57 per cent of the offspring, appears to be dominant. But, as later
evidence shows, it is recessive. The wide nostril is dominant, but so
imperfectly that only 10 per cent have a nostril above one-half open.
Let us now consider the distribution of nostril form in families
whose parents are hybrids of the first or later generation, crossed
respectively on recessives, heterozygotes, and dominants (tables
47-49).
TABLE 47.--_Distribution of frequency of the different grades of
"openness" of nostril when one parent is heterozygous and the
other recessive, i. e., with closed nostril (DR × R)._
+------+---+----------------------------+-------------------+-----+--------------------------------------+
| | | Mother. | Father. | | Grade of openness in offspring. |
|Serial|Pen+---+-----+--------------+---+----+----+-----+---+Total+----+----+----+---+---+--+---+--+--+--+
| No. |No.| | | | | | | | | gr. | | | | | | | | | | |
| | |No.| Gen.| Races. |Gr.| No.|Gen.|Race.|Gr.| | 1 | 2 | 3 | 4 | 5 | 6| 7| 8| 9|10|
+------+---+---+-----+--------------+---+----+----+-----+---+-----+----+----+----+---+---+--+---+--+--+--+
| 13 |768|298| F2 |Med. × Polish.| 2 |1689| P. |Med | 1 | 3 | 11 | 9 | 3 | 1 | 3 |..| 1|..|..|..|
| 14 |768|509| F1 | Do. | 1 |1689| P. | Do. | 1 | 2 | 12 | 5 | 6 | 1 | 1 | .| ..|..|..|..|
| | | | | +----+----+----+---+---+--+---+--+--+--+
| | | | | Totals (53) | 23 | 14 | 9 | 2 | 4 | 0| 1 |..|..|..|
| | | | | Percentages |43.4|26.4|17.0|3.8|7.6|..|1.9|..|..|..|
+------+---+---+-----+--------------------------------------------+----+----+----+---+---+--+---+--+--+--+
The study of the tables 45 to 54 establishes the following
conclusions:
First, high nostril is dominant. This means that there is a factor
that inhibits the development of the narial flap. In the absence of
such a factor the flap goes on developing normally. This hypothesis
is opposed to the conclusion that I reached in my report of 1906 (pp.
68, 69). I there said:
A close agreement exists between the percentage obtained in
each generation and the expectation of the Mendelian theory,
assuming that narrow nostril is dominant. The statistics do
not, however, tell the whole story. In 36 per cent of the
cases in the F1 generation the nostril was wider than in the
"narrow" ancestor. Even in the F2 generation nearly half of the
"narrow and intermediate" were of the intermediate sort. This
intermediate form is evidence that dominance is imperfect and
segregation is incomplete.
TABLE 48.--_Distribution of frequency of grades of "openness" in
offspring when both parents are heterozygous (DR × DR)._
+------+-----+----------------------------------+----------------------------+----+-------------------------------------------------------+
| | | Mother. | Father. | | Grade of openness in offspring. |
|Serial| Pen +------+-----+---------------+-----+------+-----+---------------+----+Total+----+----+----+----+----+----+----+----+----+----+
| No. | No. | | | | | | | | | gr. | | | | | | | | | | |
| | | No. | Gen.| Races. | Gr. | No. | Gen.| Races. | Gr.| | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+------+-----+------+-----+---------------+-----+------+-----+---------------+----+-----+----+----+----+----+----+----+----+----+----+----+
| 15 | 802 | 5314 | F1 | Polish × Min | 3 | 6652 | F1 | Polish × Min | 4 | 7 | 1 | 5 | 5 | .. | .. | 1 | .. | .. | 3 | 1 |
| 16 | 805 | 5307 | F1 | Do. | 5 | 4799 | F1 | Do. | 2 | 7 | 7 | 7 | 13 | 3 | 7 | 1 | .. | 2 | 2 | 1 |
| 17 | 852 | 5104 | F1 | Hou. × Dk. Br | 3 | 5969 | F1 | Hou. × Dk. Br | 3 | 6 | 4 | 11 | 4 | 2 | 1 | 1 | 1 | .. | .. | .. |
| 18 | 805 | 4800 | F1 | Polish × Min | 3 | 4799 | F1 | Polish × Min | 2 | 5 | 10 | 13 | 9 | 1 | 2 | 8 | .. | 1 | 2 | .. |
| 19 | 805 | 5308 | F1 | Do. | 3 | 4799 | F1 | Do. | 2 | 5 | 3 | 7 | 3 | 2 | 1 | .. | .. | .. | .. | .. |
| 21 | 759 | 797 | F1 | Houd. × Min | 3 | 570 | F1 | Houd. × Min | 2 | 5 | 2 | 4 | 2 | 2 | .. | .. | .. | .. | .. | 2 |
| 22 | 759 | 797 | F1 | Do. | 3 | 352 | F1 | Do. | 1 | 4 | .. | 2 | 2 | .. | .. | .. | .. | .. | 1 | 1 |
| 23 | 805 | 4447 | F1 | Polish × Min. | 2 | 4799 | F1 | Polish × Min | 2 | 4 | 6 | 5 | 4 | .. | 2 | .. | 1 | 1 | 3 | .. |
| 24 | 805 | 4765 | F1 | Do. | 2 | 4799 | F1 | Do. | 2 | 4 | 5 | 12 | 4 | 2 | 1 | 1 | 2 | .. | 2 | .. |
| 25 | 805 | 4797 | F1 | Do. | 2 | 4799 | F1 | Do. | 2 | 4 | 4 | 2 | 6 | .. | .. | .. | .. | 1 | .. | .. |
| 26 | 805 | 5163 | F1 | Do. | 2 | 4799 | F1 | Do. | 2 | 4 | 7 | 17 | 13 | 4 | 1 | 2 | 2 | 2 | 1 | .. |
| 27 | 805 | 5304 | F1 | Do. | 2 | 4799 | F1 | Do. | 2 | 4 | 5 | 9 | 8 | .. | 1 | .. | .. | .. | .. | .. |
| 28 | 852 | 7070 | F1 | Hou. × Dk. Br | 1 | 5969 | F1 | Hou. × Dk. Br | 3 | 4 | 4 | 11 | 4 | 2 | 1 | 1 | 1 | .. | .. | .. |
| 29 | 759 | 529 | F1 | Houd. × Min | 2 | 570 | F1 | Houd. × Min | 2 | 4 | 2 | 3 | .. | .. | .. | .. | .. | .. | .. | 1 |
| 30 | 759 | 529 | F1 | Do. | 2 | 352 | F1 | Do. | 2 | 4 | 1 | 3 | .. | .. | .. | .. | .. | .. | .. | .. |
| 31 | 728 | 174 | F1 | Hou. × Wh. L | 1 | 258 | F1 | Hou. × Wh. L | 2 | 3 | 2 | 7 | 2 | 1 | 1 | 1 | 1 | .. | .. | .. |
| 32 | 805 | 4798 | F1 | Polish × Min | 1 | 4799 | F1 | Polish × Min | 2 | 3 | 7 | 10 | 3 | 2 | 1 | 2 | .. | 4 | 2 | .. |
| 33 | 805 | 5323 | F1 | Do. | 1 | 4799 | F1 | Do. | 2 | 3 | 17 | 7 | 2 | .. | .. | 1 | .. | 2 | 1 | .. |
| | | | | +----+----+----+---------+----+----+----+----+----+
| | | | | Totals (435) | 92 |147 | 88 | 21 | 22 | 19 | 10 | 13 | 17 | 6 |
| | | | | Percentages |21.2|33.8|20.2|4.8 |5.0 |4.4 |2.3 |3.0 |3.9 |1.4 |
| | | | | |\-----------------/|\---------------------------/|
| | | | | | 80.0 | 20.0 |
+------+-----+------+-----+-------------------------------------------------------------+-------------------+-----------------------------+
TABLE 49.--_Distribution of frequency of grades of "openness" in
offspring when both parents are heterozygous (DR × DR, F2 and
later generations)._
+------+-----+---------------------------------+--------------------------------+-----+-------------------------------------------------+
| | | Mother. | Father. | | Grade of openness in offspring. |
|Serial| Pen +------+----+---------------+-----+------+----+---------------+----+Total+----+----+----+----+----+----+----+----+----+----+
| No. | No. | | | | | | | | | gr. | | | | | | | | | | |
| | | No. |Gen.| Races. | Gr. | No. |Gen.| Races. | Gr.| | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+------+-----+------+----+---------------+-----+------+----+---------------+----+-----+----+----+----+----+----+----+----+----+----+----+
| 34 | 763 | 3799 | F2 | Hou. × Wh. L. | 6 | 2247 | F2 | Hou. × Wh. L. | 2 | 8 | .. | 2 | 2 | 2 | 2 | .. | 1 | .. | .. | .. |
| 35 | 765 | 84 | F1 | Do. | 3 | 1794 | F2 | Do. | 5 | 8 | 1 | 6 | 8 | 1 | 4 | 1 | 1 | 2 | 6 | 3 |
| 36 | 765 | 984 | F2 | Do. | 3 | 1794 | F2 | Do. | 5 | 8 | 8 | 3 | 1 | 2 | 2 | 1 | 0 | 2 | 5 | .. |
| 37 | 802 | 4013 | F3 | Polish × Min. | 4 | 6652 | F1 | Polish × Min. | 4 | 8 | 6 | 12 | 9 | 6 | .. | .. | .. | 1 | 1 | 1 |
| 38 | 802 | 3954 | F3 | Do. | 3 | 6652 | F1 | Do. | 4 | 7 | 4 | 12 | 3 | 2 | 1 | .. | 2 | 6 | 9 | 1 |
| 39 | 802 | 4038 | F2 | Do. | 3 | 6652 | F1 | Do. | 4 | 7 | 3 | 8 | 4 | 3 | 2 | .. | 1 | 4 | 1 | 1 |
| 40 | 802 | 4164 | F3 | Do. | 3 | 6652 | F1 | Do. | 4 | 7 | 6 | 8 | 6 | 2 | 1 | 1 | .. | 2 | 2 | 2 |
| 41 | 812 | 84 | F1 | Hou. × Wh. L. | 3 | 4118 | F3 | Hou. × Wh. L. | 4 | 7 | .. | 1 | 5 | 2 | 2 | 1 | 1 | 1 | 1 | 2 |
| 42 | 812 | 913 | F2 | Do. | 3 | 4118 | F3 | Do. | 4 | 7 | 10 | 6 | 6 | 1 | .. | .. | .. | 3 | 2 | 5 |
| 43 | 812 | 4728 | F3 | Do. | 3 | 4118 | F3 | Do. | 4 | 7 | 8 | 5 | 5 | 1 | 5 | 2 | 2 | 2 | 9 | 2 |
| 44 | 812 | 5120 | F3 | Do. | 3 | 4118 | F3 | Do. | 4 | 7 | 1 | 2 | 2 | .. | 1 | .. | .. | .. | 1 | 2 |
| 45 | 812 | 5540 | F3 | Polish × Min. | 3 | 4118 | F3 | Do. | 4 | 7 | 2 | 5 | 6 | 1 | 1 | .. | .. | .. | .. | .. |
| 46 | 763 | 2250 | F3 | Hou. × Wh. L | 5 | 2247 | F2 | Do. | 2 | 7 | 4 | 10 | 2 | .. | .. | .. | .. | 1 | 0 | 2 |
| 47 | 812 | 4726 | F2 | Do. | 2 | 4118 | F3 | Polish × Min. | 4 | 6 | 4 | 6 | 3 | .. | 2 | 1 | .. | 2 | 1 | 3 |
| 48 | 812 | 4735 | F3 | Do. | 2 | 4118 | F3 | Do. | 4 | 6 | 2 | 1 | 1 | .. | .. | .. | .. | 2 | 1 | .. |
| 49 | 765 | 1790 | F3 | Do. | 1 | 1794 | F2 | Hou. × Wh. L. | 5 | 6 | 9 | 14 | 9 | 1 | 3 | 0 | 2 | 0 | 3 | .. |
| 50 | 802 | 4012 | F3 | Polish × Min. | 1 | 6652 | F1 | Polish × Min. | 4 | 5 | 5 | 13 | 11 | 3 | 2 | .. | 1 | 3 | 1 | .. |
| 51 | 825 | 2198 | F3 | Do. | 3 | 3852 | F3 | Do. | 2 | 5 | .. | .. | 1 | 3 | .. | .. | .. | .. | .. | 1 |
| 52 | 728 | 2271 | F2 | Hou. × Wh. L. | 3 | 258 | F1 | Hou. × Wh. L. | 2 | 5 | 4 | 3 | 1 | 7 | 2 | 1 | 3 | 1 | 1 | 2 |
| 53 | 763 | 2700 | F2 | Do. | 3 | 2247 | F2 | Do. | 2 | 5 | 1 | 2 | 3 | 3 | .. | 1 | .. | .. | 2 | .. |
| 54 | 825 | 350 | F1 | Polish × Min. | 2 | 3852 | F3 | Polish × Min. | 2 | 4 | 4 | 13 | 6 | 4 | .. | .. | .. | 3 | 1 | 3 |
| 55 | 825 | 4708 | F3 | Do. | 2 | 3852 | F3 | Do. | 2 | 4 | 4 | 13 | 7 | 3 | .. | 1 | 1 | 1 | 2 | 3 |
| 56 | 825 | 5019 | F2 | Do. | 2 | 3852 | F3 | Do. | 2 | 4 | 1 | 1 | .. | .. | .. | .. | .. | 1 | 2 | 2 |
| 57 | 825 | 5035 | F3 | Do. | 2 | 3852 | F3 | Do. | 2 | 4 | 4 | .. | 3 | 1 | 1 | .. | .. | 1 | 1 | 1 |
| 58 | 825 | 5672 | F3 | Do. | 2 | 3852 | F3 | Do. | 2 | 4 | 1 | 3 | 2 | .. | 2 | .. | .. | 1 | 2 | 1 |
| 59 | 728 | 2248 | F2 | Hou. × Wh. L. | 2 | 258 | F1 | Hou. × Wh. L. | 2 | 4 | 3 | 6 | 7 | 2 | .. | .. | 1 | 0 | 1 | 3 |
| 61 | 763 | 377 | F1 | Do. | 1 | 2247 | F2 | Do. | 2 | 3 | 20 | 9 | 14 | 3 | 6 | 0 | 2 | 0 | 2 | 1 |
| | | | | +----+----+----+----+----+----+----+----+----+----+
| | | | | Totals (663) |115 |641 | 127| 53 | 39 | 10 | 8 | 39 | 57 | 41 |
| | | | | Percentages |17.4|24.7|19.2| 8.0|5.9 |1.5 |2.7 |5.9 |8.6 |6.2 |
| | | | | |\-----------------/|\---------------------------/|
| | | | | | 69.3 30.7 |
+------+-----+------+----+------------------------------------------------------------+-------------------+-----------------------------+
These earlier data were not even roughly quantitative, and it is the
quantitative data that first give the key to the true relations.
However, sufficient evidence for the change in the conclusion is
certainly due. The evidence is found in a careful study of table
55, keeping constantly in mind this fundamental principle that the
recessive condition alone in the parents can never give rise to
the dominant; for the recessive condition implies entire absence
of the dominant factor. But the pure dominant condition will vary
in the direction of the recessive condition; such a result implies
only a partial failure of the factor to develop completely; and we
should not be surprised if occasionally the failure were complete.
This implies no "reversal of dominance," but rather an arrested
development of the factor.
TABLE 50.--_Distribution of frequency of grades of "openness" in
offspring when one parent is heterozygous and the other an
original dominant (DR × D, originals)._
+------+----+---------------------------------+---------------------+-----+-------------------------------------------------+
|Serial| Pen| Mother. | Father. |Total| Grade of openness in offspring. |
| No. | No.+-----+----+------------------+---+-----+----+------+---+ gr. |----+----+----+----+----+----+----+----+----+----+
| | | No. |Gen.| Races. |Gr.| No. |Gen.|Races.|Gr.| | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+------+----+-----+----+------------------+---+-----+----+------+---+-----+----+----+----+----+----+----+----+----+----+----+
| 62 | 803| 529| F1 | Houd. × Min. | 3 | 7522| P. | Houd.| 9 | 12 | 4 | 2 | 4 | 1 | 2 | .. | .. | 2 | 2 | 1 |
| 63 | 803| 7065| F1 | Houd. × Dk. Brah.| 1 | 7522| P. | Do. | 9 | 10 | 6 | 11 | 6 | 4 | 2 | 1 | 2 | 6 | 4 | 1 |
| | | | | +----+----+----+----+----+----+----+----+----+----+
| | | | | Totals (61) | 10 | 13 | 10 | 5 | 4 | 1 | 2 | 8 | 6 | 2 |
| | | | | Percentages |16.4|21.3|16.4| 8.2| 6.5| 1.6| 3.3|13.1| 9.8| 3.3|
| | | | | |\--------\/-------/|\-------------\/------------/|
| | | | | | 62.3 | 37.7 |
+------+----+-----+----+--------------------------------------------------+-------------------+-----------------------------+
TABLE 51.--_Distribution of frequency of grades of "openness" in
offspring when one parent is heterozygous and the other an
extracted dominant (DR × DD, extracted)._
[ABBREVIATIONS: H = Houdan; L = Leghorn; M = Minorca; P = Polish; WL = White Leghorn.]
+------+----+------------------------+----------------------+-----+-------------------------------------------------+
|Serial| Pen| Mother. | Father. |Total| Grade of openness in offspring. |
| No. | No.+-----+----+---------+---+-----+----+-------+---+ gr. +----+----+----+----+----+----+----+----+----+----+
| | | No. |Gen.| Races. |Gr.| No.|Gen.| Races.|Gr.| | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+------+----+-----+----+---------+---+-----+----+-------+---+-----+----+----+----+----+----+----+----+----+----+----+
| 64 | 832| 4404| F3 | H × WL | 4| 5119| F3 | H × WL| 10| 14 | 1 | 1 | 1 | 2 | .. | .. | .. | 8 | 1 | 1 |
| 65 | 729| 913| F2 | Do. | 6| 936| F2 | Do. | 10| 16 | 5 | 6 | 16 | 2 | 5 | .. | 3 | 11 | 11 | 10 |
| 66 | 819| 57| F1 | P × M | 4| 1420| F2 | P × M | 10| 14 | 3 | 2 | 4 | .. | .. | 1 | .. | 3 | 5 | 1 |
| 67 | 832| 505| F1 | (H × L)L| 4| 5119| F3 | H × WL| 10| 14 | 2 | 2 | 3 | 3 | 2 | .. | 2 | 2 | 2 | 4 |
| 68 | 729| 935| F2 | H × WL | 4| 936| F2 | Do. | 10| 14 | 3 | 5 | 4 | 0 | 3 | 2 | 5 | 12 | 15 | 3 |
| 69 | 756| 2011| F2 | HPMWL | 4| 444| F2 | Do. | 10| 14 | .. | .. | .. | 1 | .. | 1 | .. | 1 | 4 | 3 |
| 70 | 807| 185| F1 | P × M | 4| 3894| F3 | P × M | 9| 13 | 4 | 2 | .. | .. | .. | 2 | 1 | 1 | 2 | 1 |
| 71 | 756| 1048| F2 | Do. | 3| 1390| F2 | Do. | 10| 13 | .. | .. | 3 | .. | .. | .. | .. | .. | 2 | .. |
| 72 | 762| 505| .. | (H × L)L| 3| 444| F2 | H × L | 10| 13 | 1 | 1 | 3 | 1 | 1 | 2 | 2 | 2 | 3 | 4 |
| 73 | 762| 2011| F3 | HPML | 4| 2621| F2 | HPML | 9| 13 | .. | 1 | .. | .. | .. | .. | 1 | 1 | 3 | 1 |
| 74 | 813| 2271| F2 | H × WL | 5| 3904| F3 | H × WL| 7| 12 | 1 | 5 | 5 | 2 | 2 | 1 | 3 | 2 | 4 | 9 |
| 75 | 820| 984| F2 | H × L | 3| 4731| F3 | P × M | 9| 12 | .. | 5 | 4 | 2 | 5 | 1 | .. | 5 | 5 | 4 |
| 76 | 728| 2272| F2 | Do. | 10| 258| F1 | H × L | 2| 12 | 2 | 7 | 9 | 4 | 4 | 3 | 2 | 7 | 7 | 9 |
| 77 | 756| 1043| F2 | P × M | 2| 1390| F2 | P × M | 10| 12 | 5 | 5 | 3 | 2 | .. | .. | .. | 3 | 2 | 2 |
| 78 | 762| 505| .. | (H × L)L| 3| 2621| F3 | HPML | 9| 12 | 1 | .. | .. | .. | .. | 1 | .. | .. | .. | 3 |
| 79 | 803| 2250| F2 | H × L | 3| 7522| P. | Houd. | 9| 12 | .. | 5 | 2 | 2 | 4 | .. | .. | 4 | 9 | 6 |
| 80 | 803| 2254| F2 | Do. | 3| 7522| P. | Do. | 9| 12 | 6 | 6 | 4 | 1 | 2 | 1 | 1 | 3 | 6 | 3 |
| 81 | 769| 492| F1 | Do. | 2| 911| F2 | H × L | 9| 11 | 3 | 6 | 1 | 1 | .. | 2 | .. | .. | 1 | .. |
| 82 | 807| 1043| F2 | P × M | 2| 3894| F3 | P × M | 9| 11 | 9 | 4 | 2 | .. | 3 | 3 | .. | 6 | 6 | .. |
| 83 | 769| 2254| F2 | H × L | 1| 911| F2 | H × L | 9| 10 | 7 | 7 | 2 | 1 | .. | .. | 1 | 2 | 4 | 1 |
| 84 | 813| 935| F2 | Do. | 3| 3904| F3 | Do. | 7| 10 | 1 | 2 | .. | 4 | 4 | 3 | .. | 7 | 8 | 1 |
| 85 | 813| 5113| F3 | Do. | 3| 3904| F3 | Do. | 7| 10 | 4 | 5 | 5 | .. | 1 | 1 | 1 | 6 | 8 | 5 |
| 86 | 813| 5142| F3 | Do. | 3| 3904| F3 | Do. | 7| 10 | .. | 2 | .. | .. | .. | .. | .. | 1 | 1 | 3 |
| 87 | 813| 5122| F3 | Do. | 2| 3904| F3 | Do. | 7| 9 | .. | 1 | 2 | .. | 1 | .. | .. | 2 | 2 | 3 |
| 88 | 813| 7320| F3 | Do. | 2| 3904| F3 | Do. | 7| 9 | .. | 6 | 1 | .. | 1 | .. | .. | 2 | 5 | 2 |
| 89 | 813| 377| F1 | Do. | 1| 3904| F3 | Do. | 7| 8 | 10 | .. | 6 | 1 | .. | .. | 1 | 4 | 3 | .. |
| | | | | +----+----+----+----+----+----+----+----+----+----+
| | | | | Totals (641) | 68 | 86 | 80 | 29 | 38 | 24 | 23 | 95 |119 | 79 |
| | | | | Percentages |10.6|13.4|12.5| 4.5| 5.9| 3.7| 3.6|14.8|18.6|12.3|
| | | | | |\--------\/-------/|\-------------\/------------/|
| | | | | | 41.0 | 59.0 |
+------+----+-----+----+------------------------------------------+-------------------+-----------------------------+
TABLE 52.--_Distribution of frequency of grades of "openness" in
offspring when both parents are extracted dominants (extracted
DD × DD)._
[ABBREVIATIONS: H = Houdan; L = Leghorn; M = Minorca; P = Polish;
WL = White Leghorn.]
+------+----+---------------------+---------------------+-----+-------------------------------------------------+
|Serial| Pen| Mother. | Father. |Total| Grade of openness in offspring. |
| No. | No.+-----+----+------+---+-----+----+------+---+ gr. +----+----+----+----+----+----+----+----+----+----+
| | | No. |Gen.|Races.|Gr.| No.|Gen.|Races.|Gr.| | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+------+----+-----+----+------+---+-----+----+------+---+-----+----+----+----+----+----+----+----+----+----+----+
| 91 | 729| 2016| F2 | HPLM | 10| 936| F2 | H × L| 10| 20 | .. | .. | .. | .. | .. | .. | .. | 4 | 6 | 5 |
| 92 | 729| 2255| F2 | H × L| 10| 936| F2 | Do. | 10| 20 | 3 | 3 | .. | .. | .. | 1 | 2 | 5 | 11 | 10 |
| 93 | 729| 2269| F2 | Do. | 10| 936| F2 | Do. | 10| 20 | .. | 1 | .. | .. | .. | 1 | .. | 3 | 9 | 13 |
| 94 | 729| 2324| F2 | HPLM | 10| 936| F2 | Do. | 10| 20 | 2 | 3 | .. | .. | 1 | 1 | .. | 5 | 16 | 7 |
| 95 | 756| 1067| F2 | P × M| 10| 1390| F2 | P × M| 10| 20 | .. | 1 | 3 | 2 | 1 | .. | .. | 1 | 1 | .. |
| 96 | 756| 1113| F2 | Do. | 10| 1390| F2 | Do. | 10| 20 | .. | .. | .. | .. | .. | .. | 1 | 4 | 8 | 4 |
| 97 | 762| 2014| F3 | HPLM | 10| 444| F2 | H × L| 10| 20 | .. | .. | .. | .. | .. | .. | .. | .. | 1 | 4 |
| 98 | 819| 1113| F2 | P × M| 10| 1420| F2 | P × M| 10| 20 | .. | .. | .. | .. | .. | .. | .. | 2 | 6 | 2 |
| 99 | 819| 4257| F3 | Do. | 10| 1420| F2 | Do. | 10| 20 | .. | .. | 2 | .. | .. | .. | .. | 4 | 4 | 3 |
| 100 | 832| 4732| F3 | H × L| 10| 5119| F3 | H × L| 10| 20 | .. | .. | .. | .. | .. | .. | .. | 2 | 1 | .. |
| 101 | 832| 6481| F3 | Do. | 10| 5119| F3 | Do. | 10| 20 | .. | .. | .. | .. | .. | .. | .. | 2 | 5 | 4 |
| 102 | 756| 369| F2 | P × M| 9| 1390| F2 | P × M| 10| 19 | .. | 2 | .. | .. | .. | .. | .. | .. | 1 | 1 |
| 103 | 762| 2618| F2 | HPLM | 9| 444| F2 | H × L| 10| 19 | .. | .. | .. | .. | .. | .. | .. | 1 | 1 | .. |
| 104 | 762| 3776| F2 | H × L| 9| 444| F2 | Do. | 10| 19 | .. | .. | .. | .. | .. | .. | .. | 1 | 1 | .. |
| 105 | 832| 5803| F3 | Do. | 9| 5119| F3 | Do. | 10| 19 | .. | .. | 1 | 1 | .. | .. | 1 | 6 | 9 | 6 |
| 106 | 807| 1067| F2 | P × M| 10| 3894| F3 | P × M| 9| 19 | .. | 1 | 1 | 1 | 2 | 1 | 2 | 1 | 4 | 2 |
| 107 | 762| 2333| F3 | HPLM | 8| 444| F2 | H × L| 10| 18 | .. | .. | .. | .. | .. | .. | 1 | 2 | 5 | 4 |
| 108 | 762| 2618| F2 | Do. | 9| 2621| F3 | HPLM | 9| 18 | .. | .. | .. | .. | .. | .. | 1 | 2 | 2 | .. |
| 109 | 762| 3776| F2 | H × L| 9| 2621| F3 | Do. | 9| 18 | .. | .. | .. | .. | 1 | .. | 2 | 4 | 4 | .. |
| 110 | 819| 5674| F2 | P × M| 8| 1420| F2 | P × M| 10| 18 | 1 | .. | 1 | .. | .. | .. | 2 | 1 | 3 | 2 |
| 111 | 820| 2016| F2 | HPLM | 9| 4731| F3 | Do. | 9| 18 | .. | .. | .. | .. | 1 | .. | 1 | 1 | 4 | .. |
| 112 | 820| 2255| F2 | H × L| 9| 4731| F3 | Do. | 9| 18 | .. | .. | .. | .. | .. | .. | 1 | 2 | 6 | 5 |
| 113 | 820| 6479| F3 | Do. | 9| 4731| F3 | Do. | 9| 18 | .. | .. | 1 | .. | 2 | 1 | 2 | 9 | 12 | 4 |
| 114 | 832| 2618| F2 | HPLM | 8| 5119| F3 | H × L| 10| 18 | 1 | 1 | 3 | 4 | .. | .. | .. | .. | 12 | 3 |
| 115 | 832| 3776| F2 | H × L| 8| 5119| F3 | Do. | 10| 18 | .. | 3 | 3 | .. | 2 | .. | .. | .. | .. | .. |
| 116 | 834| 2324| F2 | HPML | 9| 5090| F2 | Do. | 9| 18 | .. | 1 | .. | .. | .. | 1 | .. | 10 | 10 | 3 |
| 117 | 762| 2333| F3 | HPLM | 8| 2621| F3 | HPLM | 9| 17 | .. | .. | .. | .. | 1 | .. | 1 | .. | .. | 1 |
| 118 | 807| 5075| F2 | P × M| 7| 3894| F3 | P × M| 9| 16 | .. | 1 | .. | .. | 2 | 1 | .. | 5 | 7 | 7 |
| 119 | 820| 5143| F3 | H × L| 7| 4731| F3 | Do. | 9| 16 | 1 | 1 | 2 | 5 | .. | 1 | 3 | 10 | 10 | 12 |
| 120 | 813| 2272| F2 | Do. | 9| 3904| F3 | H × L| 7| 16 | 1 | 1 | 1 | .. | 1 | .. | 2 | 5 | 7 | 7 |
| | | | | +----+----+----+----+----+----+----+----+----+----+
| | | | | Totals (472) | 9 | 19 | 18 | 13 | 14 | 8 | 22 | 93 | 169| 105|
| | | | | Percentages | 1.9| 4.0| 3.8| 2.8| 3.0| 1.7| 4.7|19.8|36.0|22.3|
+------+----+-----+----+--------------------------------------+----+----+----+----+----+----+----+----+----+----+
TABLE 53.--_Distribution of frequency of grades of "openness" in
offspring when both parents are heterozygous (RR × DR)._
+------+----+--------------------------------+--------------------------------+-------------------------------------------------+
|Serial| Pen| Mother. | Father. | Grade of openness in offspring. |
| No. | No.+-----+----+--------------+------+-----+----+--------------+------+----+----+----+----+----+----+----+----+----+----+
| | | No.|Gen.| Races. |Grade.| No.|Gen.| Races. |Grade.| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+------+----+-----+----+--------------+------+-----+----+--------------+------+----+----+----+----+----+----+----+----+----+----+
| 121 | 728| 174| F1 | Houd. × Legh.| 1 | 1258| P. | Brah. × Tosa.| 2 | 2 | 7 | 2 | 1 | 1 | 1 | 1 | .. | .. | .. |
| 122 | 728| 912| F2 | Do. | 2 | 258| F1 | Houd. × Legh.| 2 | 7 | 3 | 3 | 2 | 1 | .. | .. | .. | .. | .. |
| 123 | 763| 3799| F1 | Min. × Houd. | 6 | 2247| F2 | Do. | 2 | .. | 2 | 2 | 2 | 2 | .. | 1 | .. | 2 | .. |
| 124 | 802| 509| F2 | Polish × Min.| 1 | 6652| F1 | Polish × Min | 4 | 6 | 6 | 1 | .. | 1 | .. | .. | .. | .. | .. |
| 125 | 802| 3846| F2 | Do. | 2 | 6652| F1 | Do. | 4 | 1 | 6 | 3 | 1 | 1 | .. | .. | .. | .. | .. |
| 126 | 802| 5025| F3 | Do. | 2 | 6652| F1 | Do. | 4 | 8 | 10 | 4 | 3 | 2 | .. | .. | .. | .. | .. |
| 127 | 802| 5087| F3 | Do. | 2 | 6652| F1 | Do. | 4 | 7 | 9 | 12 | 2 | .. | 1 | .. | .. | .. | 1 |
| | | | | +----+----+----+----+----+----+----+----+----+----+
| | | | | Totals (217) | 31 | 43 | 27 | 11 | 8 | 2 | 2 | 0 | 2 | 1 |
| | | | | Percentages |24.4|33.9|21.3| 8.7| 6.3| 1.6| 1.6| 0| 1.6| 0.8|
+------+----+-----+----+------------------------------------------------------+----+----+----+----+----+----+----+----+----+----+
TABLE 54.--_Distribution of frequency of grades of "openness" in
offspring when both parents are extracted recessives (extracted
RR × RR)._
+------+----+----------------------------------+--------------------------------+------+---------------+
|Serial| Pen| Mother. | Father. |Total | Offspring. |
| No. | No.+-------+----+--------------+------+-----+----+--------------+------+grade.+-------+-------+
| | | No. |Gen.| Races. |Grade.| No. |Gen.| Races. |Grade.| |Grade 1|Grade 2|
+------+----+-------+----+--------------+------+-----+----+--------------+------+------+-------+-------+
| 128 | 728| [A]912| F2 | Houd. × Legh.| 2 | 1298| F2 | Houd. × Legh.| 1 | 3 | 3 | 3 |
| 129 | 827| 298| F2 | Pol. × Min. | 2 | 3852| F3 | Do. | 2 | 4 | 5 | 5 |
+------+----+-------+----+--------------+------+-----+----+--------------+------+------+---------------+
[A] _Cf._ Serial No. 12_a._
At the outset, then, we find (table 55) that even pure races with
high nostril (Polish, Houdans), when bred together, vary much in the
height of nostril (in perfection of dominance) and, in 2 per cent of
the offspring, even show the typical narrow nostril (fig. B, _a_). On
the other hand, in the narrow-nostriled races I have never obtained
any such variation. The most deviation that I have seen from grade
1 is found in my strain of Dark Brahma bantams that frequently give
grade 2. The variability of the high nostril, the stability of the
low nostril, is _prima facie_ evidence that the former is due to the
presence of a particular factor and the latter to its absence.
[Illustration: Fig. B.--Polygons of frequency of grades of
"openness" of nostril in offspring of various parents. _a_,
Both parents pure bred dominants; _b_, both parents extracted
dominants; _c_, one parent heterozygous, the other a dominant;
_d_, both parents heterozygous; _e_, dominant by recessive; _f_,
heterozygous by recessive; _g_, heterozygous by extracted
recessive; _h_, extracted recessives; _i_, heterozygous by
dominant; _k_, both parents second generation hybrids.]
Next, the heterozygotes of F1 (table 46), may be appealed to; but
they will give no critical answer. For expectation, dominance being
imperfect, is that the hybrids will be intermediate, and the result
will be the same whichever extreme grade is taken as dominant. The
empirical mode in the distribution of the offspring is at grade 2.
This implies much greater imperfection of dominance on the hypothesis
that grade 10 is dominant than on the hypothesis that grade 1 is
dominant; but this very fact supports the former hypothesis, since
imperfection of dominance is obviously a feature of the character
with which we are dealing.
The critical test is afforded by the F2 generation (tables 48 and
49). By hypothesis, 25 per cent of the offspring are expected to be
pure ("extracted") recessives, and the same number pure dominants;
and also, by hypothesis, the recessives are massed at or near one
grade while the dominants are variable. Now, as a matter of fact,
the upper 25 per cent range over 5 to 7 grades, while the lower 25
per cent are nearly massed in grade 1 (21 per cent are so massed in
one table, 17 per cent in the other). Therefore, in accordance with
hypothesis we must regard the lower grade--narrow slit--as recessive.
Similarly, heterozygous × low nostril (table 47) should give, on our
hypothesis, 50 per cent low nostril. If that is recessive we should
expect a massing of this 50 in the first two grades; if dominant a
greater scattering. The former alternative is realized. Again, in the
heterozygous × high nostril hybrid (table 50) the upper 50 per cent
will be massed or scattered according as high nostril is recessive
or dominant. Allowing for the 50 per cent heterozygotes in the
progeny, the 50 per cent of high nostrils are scattered through at
least 8 grades of the possible 10. High nostril is dominant. Finally,
extracted high nostrils bred together produce offspring (table
52) with a great range of variability (through all grades), while
extracted low nostrils (unfortunately all too few) give progeny with
grades 1 and 2 (table 53; fig. B, _h_). Accepting, then, the general
principle of the greater variability of the dominant character, we
have demonstrated conclusively that high nostril, or rather the
factor that determines high nostril, is dominant.
Comparing tables 45 to 54, we see that recessive parents are
characterized by a low grade of nostril and they, of course, tend
to produce offspring with a low grade. Similarly, dominants have a
high grade and tend to produce offspring of the same sort, while
heterozygous parents are of intermediate grade and their children
have nostril grades that are, on the average, intermediate. Without
regarding the gametic constitution, we might conclude, with Castle,
that offspring inherit the grade of their parents, and consequently
it would be possible to increase the grade, perhaps indefinitely, by
breeding from parents with the highest grade. Considering the gametic
constitution of the parents, it is obvious that such a conclusion
is premature. To get an answer to the question it is necessary to
find if there is, inside of any one table, among parents of the same
gametic constitution, any such relation between parental and filial
grades. This can be determined by calculating the correlation between
the grades of parents and progeny. Such calculation I have made for
table 48 with the result: index of correlation, _r_ = 0.018 ± 0.032,
which is to be interpreted as indicating that no correlation exists;
and in so far the hypothesis of Castle proves not to apply in the
cases of booting and doubt is thrown on the significance of his
conclusion.
Finally, if we throw together the frequency distributions of all
tables into one table (table 55; compare fig. B) we shall find the
totals instructive. Table 55 shows that, when all results are thrown
together, including hybrids of all sorts, grade 2 and grade 9 are
the most frequent and grade 6 is the least frequent, the frequency
gradually rising towards the extremes of the series. The same result
appears in the individual series that range from grade 1 to grade
10. What is the meaning of this result? It seems to me to bear but
one interpretation, namely, that there are only two centers of
stability--about grades 1 and 9--and true blending of these grades,
giving an intermediate condition, does not occur. Otherwise, in
consequence of the repeated hybridization, the intermediate grades
must be the commonest instead of the rarest. There is alternative
inheritance of the nostril height.
TABLE 55.--_Summary of tables 45 to 54._
+---------------------------------------------------------------------------------------------------------------------------+
| ABSOLUTE FREQUENCIES. |
+-------+------------------------------+----------------+-------------------------------------------------------------------+
| | | | Grade of openness in offspring. |
| Table | Nature of mating (parental | Nature of +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-------+
| No. | nostril). | mating. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Total |
+-------+------------------------------+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-------+
| 45 | High × high | D × D | 2 | 2 | 1 | 1 | 6 | 5 | 8 | 28 | 39 | 27 | 119 |
| 46 | High × low | D × R | 13 | 19 | 9 | 7 | 2 | 4 | 1 | 1 | ... | ... | 56 |
| 47 | Heterozygous × low | DR × R | 23 | 14 | 9 | 2 | 4 | ... | 1 | ... | ... | ... | 53 |
| 48 | Heterozygous × heterozygous | DR × DR | 90 | 140 | 86 | 20 | 21 | 18 | 9 | 13 | 17 | 6 | 420 |
| 49 | Do. | F2(DR × DR) | 117 | 171 | 129 | 54 | 40 | 11 | 19 | 39 | 57 | 41 | 678 |
| 50 | Heterozygous × high | DR × D | 10 | 13 | 10 | 5 | 4 | 1 | 2 | 8 | 6 | 2 | 61 |
| 51 | Do. | DR × DD | 71 | 96 | 73 | 30 | 39 | 24 | 23 | 95 | 119 | 68 | 638 |
| 52 | Extra high × high | DD × DD | 9 | 19 | 18 | 15 | 14 | 8 | 22 | 93 | 169 | 105 | 472 |
| 53 | Heterozygous × extracted low | DR × RR | 40 | 35 | 26 | 7 | 3 | 1 | ... | ... | ... | ... | 112 |
| 54 | Extra low × low | RR × RR | 8 | 8 | ... | ... | ... | ... | ... | ... | ... | ... | 16 |
| | | +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-------+
| | Totals | | 378 | 512 | 361 | 141 | 133 | 72 | 85 | 277 | 407 | 249 | ... |
+-------+------------------------------+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-------+
| PERCENTAGES. |
+-------+------------------------------+----------------+-------------------------------------------------------------------+
| | | | Grade of openness in offspring. |
| Table | Nature of mating (parental | Nature of +------+------+------+------+-----+-----+------+------+------+------+
| No. | nostril). | mating. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
+-------+------------------------------+----------------+------+------+------+------+-----+-----+------+------+------+------+
| 45 | High × high | D × D | 1.7 | 1.7 | 0.8 | 0.8 | 5.0 | 4.2 | 6.7 | 23.5 | 32.8 | 22.7 |
| 46 | High × low | D × R | 23.2 | 34.0 | 16.1 | 12.5 | 3.6 | 7.1 | 1.8 | 1.8 | ... | ... |
| 47 | Heterozygous × low | DR × R | 43.4 | 26.4 | 35.9 | 3.8 | 7.6 | ... | 1.9 | ... | ... | ... |
| 48 | Heterozygous × heterozygous | DR × DR | 21.5 | 33.3 | 20.5 | 4.8 | 5.0 | 4.3 | 2.1 | 3.1 | 4.1 | 1.2 |
| 49 | Do. | F2(DR × DR) | 17.3 | 25.2 | 19.0 | 8.0 | 5.9 | 1.6 | 2.8 | 5.8 | 8.4 | 6.1 |
| 50 | Heterozygous × high | DR × D | 16.4 | 21.3 | 16.4 | 8.2 | 6.6 | 1.6 | 3.3 | 13.1 | 9.8 | 3.3 |
| 51 | Do. | DR × DD | 11.1 | 15.1 | 11.4 | 4.7 | 6.1 | 3.8 | 3.6 | 14.9 | 18.7 | 10.7 |
| 52 | Extracted high × high | DD × DD | 1.9 | 4.0 | 3.8 | 3.2 | 3.0 | 1.7 | 4.7 | 19.7 | 35.8 | 22.2 |
| 53 | Heterozygous × extracted low | DR × RR | 35.8 | 31.3 | 23.2 | 6.3 | 2.7 | 0.9 | ... | ... | ... | ... |
| 54 | Extracted low × low | RR × RR | 50.0 | 50.0 | ... | ... | ... | ... | ... | ... | ... | ... |
+-------+------------------------------+----------------+------+------+------+------+-----+-----+------+------+------+------+
CHAPTER VIII.
CREST.
In my report of 1906 I called attention to the nature of inheritance
of the crest in the first and second generations. The result seemed
simple enough on the assumption of imperfect dominance. However, in
later generations difficulties appeared, one of which was referred
to in a lecture given before the Washington Academy of Sciences in
1907. I stated (1907, p. 182), that "when a crested bird is crossed
with a plain-headed one, and the crested hybrids are then crossed
_inter se_, the extracted recessives of the second hybrid generation
are plain-headed, to be sure, but they show a disturbance of certain
feathers." This was an illustration of the statement that recessives
which are supposed to come from two pure recessive gametes show in
their soma traces of the dominant type. Dr. W. J. Spillman, who
was present, made the suggestion that the crest is composed of two
characters, _T_ and _t_, instead of a simple element, and that
when _t_ alone is present the result will be the roughened _short_
feathers on top of the head.
Further studies demonstrate the validity of this suggestion. There
are in the crest two and probably three or more factors. There is
a factor that determines length of the feathers and a factor that
determines their erectness. There is probably also an extension
factor that controls the area that the crest occupies on the head.
Thus flatness of position dominates over its absence (or erectness).
This is seen even in the first generation. Figs. 5, 6, 8, and 17
of my report of 1906 show this very plainly. They also show that
continued growth of feather is dominant over interrupted growth. Thus
in the second hybrid generation I got birds with short and erect
feathers and one of these is shown in fig. 11 of the 1906 report.
That shortness is recessive is proved by various matings of extracted
short × short crest. Of 29 offspring none have a higher grade than 1,
grade 10 being of full length. On the other hand, two parents with
long feathers in the crest (grades 6 to 8) give 5 offspring of grade
1, 12 of grades 5 to 10, thus approaching the 1:3 ratio expected from
two DR parents. That erectness is recessive is proved by various
matings of extracted erect × erect crest. Of 25 offspring none has
a lower grade than 4 (1 case) or 5 (1 case). On the other hand, two
parents with extracted non-erect feathers give in 46 offspring 13
with feathers whose grade of erectness is 6 or higher and 33 with a
grade of 5 or below--of these half of grade 0--close to the expected
1:3. The evidence is conclusive that there are two factors in crest
that behave in Mendelian fashion--a factor determining the prolonged
growth of the feather and a factor causing the feathers to lie
repent.
The area of the head occupied by the crest is also variable. This
was estimated in tenths for each of the parents and offspring. Two
principal centers of variation appeared, at 3 and at 8, or roughly
one-third and two-thirds the full area. The results, being based
on estimates, are not wholly satisfactory, but so far as they go
they indicate that when both parents have a crest that belongs to
the lower center of variation their offspring belong chiefly if not
exclusively to that center; but when they both belong to the upper
center of variation a minority of the offspring belong to the lower
center. Provisionally it may be concluded that extensive crest is
dominant over the restricted crest or that there is an "extension
factor."
CHAPTER IX.
COMB-LOP.
In races having a large single comb this is usually erect in the
male, but in the female lops over to the right or left side of the
head. This lop is determined before hatching; indeed, in the male it
may be ascertainable only in the embryo or in the recently hatched
chick. The position of the comb is permanent throughout the life of
the pullet and hen and, if pressed to the opposite side, it quickly
returns to its original position. At one time I entertained the
hypothesis that its position was determined by the pressure of the
foot against the head while the chick was still within the shell; but
after finding the comb lying both to the right and to the left when
in contact with the foot I abandoned this hypothesis as untenable. It
seemed possible that this position is hereditary, and so data were
collected to test this hypothesis. It is not always easy to decide
definitely, even for the female, as to the direction of the lop; for
the anterior part of the comb may lop to the right, the posterior
part to the left, or _vice versa_. In that case one selects the
larger or better defined lopping portion to designate as _the_ lop.
This is usually the posterior portion of the comb. However, such
doubtful cases may be omitted from consideration here, as there are
plenty of examples of well-defined lop on both sides of the head.
TABLE 56.
+------------------------------------------------------+
| Both parents with right lop. |
+----------+-----------+-----------+-------------------+
| | | | Offspring. |
| Pen No. | No. of | No. of +---------+---------+
| | mother. | father. | Right. | Left. |
+----------+-----------+-----------+---------+---------+
| | | | | |
| 817 | 6188 | 3900 | 7 | 8 |
| 817 | 6406 | 3900 | 12 | 17 |
| 831 | 1011 | 4213 | 7 | 16 |
| 831 | 3040 | 4213 | 13 | 10 |
| 831 | 4219 | 4213 | 4 | 21 |
| 831 | 6602 | 4213 | 6 | 15 |
| 833 | 1310 | 4222 | 4 | 7 |
| 833 | 4361 | 4222 | 6 | 4 |
| 833 | 7519 | 4222 | 2 | 4 |
| 904 | 4714 | 7870 | 6 | 7 |
| | | +---------+---------+
| | | | 67 | 109 |
+----------+-----------+-----------+---------+---------+
| Both parents with left lop. |
+----------+-----------+-----------+---------+---------+
| | | | | |
| 841 | 3867 | 3890 | 3 | 9 |
| 841 | 4663 | 3890 | 9 | 7 |
| 903 | 9824 | 8463 | 6 | 5 |
| | | +---------+---------+
| | | | 18 | 21 |
+----------+-----------+-----------+---------+---------+
| Mother left lop, father right. |
+----------+-----------+-----------+---------+---------+
| | | | | |
| 831 | 1980 | 4213 | 9 | 17 |
| 904 | 3901 | 7840 | 4 | 3 |
| 904 | 7645 | 7840 | 6 | 3 |
| | | +---------+---------+
| | | | 19 | 23 |
+----------+-----------+-----------+---------+---------+
| Mother right lop, father left. |
+----------+-----------+-----------+---------+---------+
| 903 | 3946 | 8463 | 2 | 0 |
| 903 | 4079 | 8463 | 7 | 2 |
| 903 | 4082 | 8463 | 11 | 6 |
| | | +---------+---------+
| | | | 20 | 8 |
+------------------------------------------------------+
| Summary. |
+-----------------------------+------------------------+
| | Offspring. |
| Parents. +------+--------+--------+
| |Total.| Right. | Left. |
+-----------------------------+------+--------+--------+
| | |_P. ct._|_P. ct._|
|Both with right lop | 176 | 38 | 62 |
|Both with left lop | 39 | 46 | 54 |
|Mother left lop, father right| 42 | 45 | 55 |
|Mother right lop, father left| 28 | 71 | 29 |
+-----------------------------+------+--------+--------+
From table 56 it appears, summing all cases, that there are more
left-lopping offspring than right-lopping as 161 to 124 or as 56.5
per cent to 43.5 per cent and that this excess holds whether both
parents are right-lopping, or both left-lopping, or the mother
left and the father right. Only in the case when the mother is
right-lopping is there a majority of offspring of the same sort,
but here the numbers are too inconsiderable to carry much weight.
Although there is not clear evidence of any sort of inheritance,
it is probable that the position of the lop is not determined by a
single factor, but by a complex of factors.
The conclusion that right and left conditions are not simple,
alternative qualities accords with the results obtained by others.
Thus Larrabee (1906) finds that the dimorphism of the optic chiasma
of fishes (in some cases the right optic nerve being dorsal and in
others the left) is not at all inherited, but in each generation the
result is strictly due to chance. This is, perhaps, the same as my
conclusion that the hereditary factors are complex. Lutz (1908) finds
that in the mode of clasping the hands interdigitally the right thumb
is uppermost in 73 per cent of the offspring when both parents clasp
with right thumb uppermost, but in only 42 per cent of the offspring
when both parents clasp with left thumb uppermost. The mode of
clasping is inherited, but not in simple Mendelian fashion.
CHAPTER X.
PLUMAGE COLOR.
A. THE GAMETIC COMPOSITION OF THE VARIOUS RACES.
Plumage color, like hair color, varies greatly among domesticated
animals. This diversity is, no doubt, in part due to the striking
nature of color variations, but chiefly to the fact that the
requisite variations are afforded in abundance. The principal color
varieties, in poultry as in other domesticated animals, are melanism,
xanthism, and albinism. In addition, poultry show the dominant white,
or "gray" white, first recognized in poultry by Bateson and Saunders
(1902), which is also found in many mammals, as, for instance, in
goats, sheep, and cattle. Besides these uniform colors, we find
numerous special feather-patterns, such as lacing (or edging of the
feather), barring, penciling, and spangling. Also, there are special
patterns in the plumage as a whole, such as wing-bar, hackle, saddle,
breast, and top of head (crest). Now, all of these color characters
are inherited each in its own definite fashion.
In studying the color varieties of poultry we must first of all, as
in flower color (Correns, 1902), mice (Cuénot, 1903), guinea-pigs
and rabbits (Castle), various plants and animals (Bateson and his
pupils), recognize the existence of certain "factors." In poultry the
factors that I have determined are as follows:
_C_, the color factor, absence of which results in albinism.
_J_, the Jungle-fowl pattern and coloration.
_N_ (nigrum), the supermelanic factor.
_X_, the superxanthic or "buff" factor.
_W_, the graying (white) factor.
We have now to consider how these factors are combined in birds of
the different races.
1. WHITE.
_Albinos._--These seem to be of two different origins:[9] White
Cochins and white Silkies. The white Silkies that I have studied
have the gametic formula _cJnwx_; _i. e._, they have the Jungle-fowl
marking, but lack the "color enzyme," supermelanic coat, the graying
factor, and the xanthic factor.
[9] Bateson and Punnett (1908, p. 28) recognize three
"kinds" of recessive whites--that of the Silkie, that of the
Rose-comb bantams, and that of "white birds that have arisen in
the course of our experiments." White Cochins have perhaps been
one of the ancestors of Rose-comb bantams; Bateson's new white
lay recessive in the White Dorking and when mated to the White
Silkie throws Game-colored offspring.
"_Grays._"--White Leghorns and their derivatives belong to this
class. Its gametic formula is: _CJNWx_. This indicates that the race
contains the color enzyme, as well as the Jungle pattern and the
supermelanic coat. But all of these are rendered invisible by the
graying factor _W_. The superxanthic factor is missing.
2. BLACK.
The uniform black birds that I have studied are of several sorts.
The Black Minorca and White-faced Black Spanish have the gametic
formula _CJNwx_. Owing to the absence of the graying factor and the
presence of the color factor these appear as pigmented birds, but the
supermelanic coat, _N_, obscures the Jungle coloration, so that the
bird appears entirely black. Nevertheless the black is not of uniform
quality, but just those parts of the feathers of the wing, back,
hackle, saddle, and breast that are red in the Jungle fowl are of an
iridescent black, while the portion that is not red in the Jungle is
of a dead black.
The Black Cochin has the gametic formula _CINwx_. This differs from
the formula of the Minorca only in this respect: the Jungle pattern
is present, but not the pigmentation that is usually associated with
it.
The Black Game ("Black Devil") that I used in a few experiments
seemed to have the same gametic formula as the Minorca, only the
supermelanic coat was less dense.
3. BUFF.
For this color I used Buff Cochins, the original buff race. The
gametic formula of this race proves to be _CjnwX_--the Jungle-fowl
pattern being absent.
B. EVIDENCE.
The evidence for the gametic interpretations of the self-colored fowl
is derived from hybridizations. It will now be presented in detail.
1. SILKIE × MINORCA (OR SPANISH).
(Plates 3 to 6.)
By hypothesis this cross is between _cJnwx_ and _CJNwx_. The first
generation should give the zygotic formula _CcJ2Nnw2x2_, or, more
simply, _CcJ2Nn_. This formula resembles closely that for the
Minorca; but it differs in this important respect, that the coloring
factor and the supermelanic factor are both heterozygous, and hence
diluted.
Actually I found, as Darwin (1876) did, that the chicks of this
first hybrid generation were all wholly black. In this respect
they differed markedly from the chicks of the Silkie, which are
pure white, and also from the chicks of the Minorca, which are
prevailingly black, but have white belly and outer primaries. The
white in the young chicks of Minorcas is extremely variable in
amount, but never wholly absent; in time, as the bird grows older, it
is replaced by black, so that the adult male and female Minorcas have
a wholly black plumage. The reason for the precocious development of
black pigment over the belly and primaries of the hybrid chicks is
probably the presence of an extension factor (_cf._ Castle, 1909)
derived from the Silkie. Certain it is that the ordinary Jungle
pattern develops pigment on the belly and on the wings, as well as
on other parts of the plumage. The hybrid chicks may be said to have
the extended pigmentation dominant over interrupted pigmentation. In
the adult hybrids a difference appears between the coloration of the
male and female, even as Darwin pointed out. For the latter retains
its uniform blackness, while the former gains red on the wing-bar,
and saddle and hackle lacing (plate 4). Now, since all the factors
present in the Minorca, and none others, are present in the hybrids,
why should the male hybrids show red, and why should the males show
red and not the females? The answer to the first question is, I
think, clear. While the Jungle pattern of black and red is completely
obscured by the undiluted _N_ factor of the Minorca, it is only
incompletely covered by the diluted, heterozygous _N_ factor of the
hybrid. Hence the red appears in greatly reduced amount, as compared
with the Jungle-fowl. In the female Jungle-fowl there is little red
and consequently none shows in the female hybrid. Thus the difference
in the sexes of the hybrids corresponds to the sexual dimorphism of
the Jungle-fowl; but the hybrids are, as indicated, very unlike the
Jungle-fowl in coloration (_cf._ plates 1 and 2).
Since segregation takes place in the gametes of these heterozygotes,
4 kinds of gametes are possible, namely, _CJN_, _CJn_, _cJN_, _cJn_.
On mating heterozygotes together, zygotes of 16 types will be formed,
as in table 57.
TABLE 57.--_Zygotes in F2 of Silkie × Minorca hybrids and their
corresponding somatic colors._
+------------+-----------+-----------+-----------+
| C2J2N2 N | C2J2Nn N | CcJ2N2 N | CcJ2Nn N |
| C2J2Nn N | C2J2n2 G | CcJ2Nn N | CcJ2n2 G |
| CcJ2N2 N | CcJ2Nn N | c2J2N2 W | c2J2Nn W |
| CcJ2Nn N | CcJ2n2 G | c2J2Nn W | c2J2n2 W |
+------------+-----------+-----------+-----------+
TABLE 58.
+---------+-----------------------+-----------------------+-----------------------+
| | Black. | White. | Game. |
| Pen No. +-----------+-----------+-----------+-----------+-----------+-----------+
| | Observed. | Expected. | Observed. | Expected. | Observed. | Expected. |
+---------+-----------+-----------+-----------+-----------+-----------+-----------+
| 709 | 119 | 116 | 55 | 51 | 31 | 38 |
| 804 | 91 | 89 | 40 | 39 | 26 | 29 |
| +-----------+-----------+-----------+-----------+-----------+-----------+
| Total. | 210 | 205 | 95 | 90 | 57 | 67 |
+---------+-----------+-----------+-----------+-----------+-----------+-----------+
In the foregoing table there is given after each combination a
letter: _N_ standing for black, the appearance of the soma; _G_
standing for Game-colored, and _W_ standing for white. No distinction
is made between pure blacks and those that, while black as chicks,
subsequently show some red in the male. Such a distinction was
impracticable because most of the color determinations are made
on the young chicks. It appears that in 16 progeny expectation is
9 black, 4 white, and 3 Game-colored. Actually 362 offspring were
obtained, with the results shown in table 58. Nothing is more
striking than to see the hens of this F2 generation with evidences of
the female Game pattern (plate 6).
Comparing observed results in the distribution of colors in the F2
generation with expectation, it is seen that the proportions are
close, and this closeness of observation with expectation is evidence
for the correctness of the hypothesis.
The hypothesis may be further tested in later generations by breeding
together the different sorts of individuals obtained in F2. In
pursuance of such a test I mated various pure black hens with pure
black cocks and those of F1, and, as was to have been expected,
obtained families of different sorts, simply because even pure blacks
have differing gametic constitutions. Thus in pen 824 I mated an
extracted black cock with 3 black hens. All were apparently of the
zygotic constitution _C2J2Nn_, forming gametes _CJN_ and _CJn_. Mated
together these should give the three black combinations _C2J2N2_,
_C2J2Nn_, _C2J2nN_, to one Game, _C2J2n2_. Actually there were
obtained 64 black and 23 Game, 66 to 22 being expectation. In another
pen (pen 804) an F1 cock was mated to various black F2 hens. The
families fall into 2 classes. The cock, of course, produced gametes
_CJN_, _CJn_, _cJN_, _cJn_. With four females like him (Nos. 3902,
3908, 5431, 6043) I got: black 40, white 13, Game 14; expected,
black 38, white 17, Game 13. Three females (Nos. 4715, 4716, 5099)
evidently produced gametes _CJN_, _CJn_. Expectation is that blacks
and Games shall be produced in the proportions of 3 to 1. Actually
30:14 were obtained where 33:11 was expected. All of these results
accord closely with the hypothesis.
The whites obtained in F2 are of 3 types, but in all alike the color
factor is missing. Hence it can not reappear in the offspring, and,
consequently, no colored offspring are to be expected. But, first,
it must be stated that the extracted whites of the F2 generation
are not always of a pure white. Indeed, the parent Silkies are in
some cases not perfectly white, but show traces of "smoke." There
are different degrees of albinism; the coloring enzyme may be absent
to small traces. This variability in degree of albinism is familiar
to all students of albinism in man. My breeding of extracted whites
was done in pen 817 and consisted of a pure white cock (No. 3900)
and 2 hens. Of these 1 (No. 6046) was pure white and produced in a
total of 15 only white offspring, but among those that were described
as unhatched I have recorded traces of pigment in 24 per cent of
the cases. The second hen (No. 3899) had black flecks in the white
plumage. She had 20 offspring, of which 2 (unhatched) are recorded
as having N down, 2 as "blue," and 3 others show traces of black
pigment. Thus, 7 birds in 20, or 35 per cent of all, show more or
less black, even as the albinic mother does. On the whole, however,
omitting from present consideration the phenomenon of incomplete
albinism, we may say that 2 pure albino parents produce only albinic
offspring, while imperfectly albinic parents produce some imperfectly
albinic offspring.
2. SILKIE × WHITE LEGHORN.
By hypothesis this cross is between _cJnwx_ and _CJNWx_. The first
generation should give the zygotic formula _CcJ2NnWwx2_, or, more
simply, _CcJ2NnWw_. This formula resembles closely that of the White
Leghorn, except that the coloring and graying factors and that for
supermelanism are all heterozygous and hence diluted; only the Jungle
coloration remains unchanged. Actually, the first generation yielded
a lot of white birds like the Leghorn, but with this difference,
that, as the males became mature, they gained red on the wing-bar and
to a slight extent on the lacing of the saddle. The females gained a
faint blush of red on the breast. Thus red appeared, in small amount,
in just those places in the respective sexes which are red in the
Jungle-fowl. The explanation of its appearance that I have to suggest
is that, both on account of the diluting of the supermelanic coat and
of the graying factor, the red of the undiluted underlying Jungle
coloration is revealed.
Since the hybrids are heterozygous in respect to 3 pairs of
characters, when segregation occurs each parent produces 8 kinds of
gametes, as follows: _CJNW_, _CJNw_, _CJnW_, _CJnw_, _cJNW_, _cJNw_,
_cJnW_, _cJnw_. When both parents produce these 8 kinds of gametes
we may expect, in 64 offspring, the proportions of the several types
shown in table 59.
TABLE 59.--_Probable frequency in 64 progeny._
+-----------------+------+------+------+------+
| Zygotic formula.|White.|White | Game.|Black.|
| | |+ red.| | |
+-----------------+------+------+------+------+
| | | | | |
| C2J2N2W2 | 1 | .. | .. | .. |
| C2J2N2Ww | 2 | .. | .. | .. |
| C2J2N2w2 | .. | .. | .. | 1 |
| C2J2NnW2 | 2 | .. | .. | .. |
| C2J2NnWw | .. | 4 | .. | .. |
| C2J2Nnw2 | .. | .. | 2 | .. |
| C2J2n2W2 | 1 | .. | .. | .. |
| C2J2n2Ww | .. | 2 | .. | .. |
| C2J2n2w2 | .. | .. | 1 | .. |
| CcJ2N2W2 | 2 | .. | .. | .. |
| CcJ2N2Ww | 4 | .. | .. | .. |
| CcJ2N2w2 | .. | .. | .. | 2 |
| CcJ2NnW2 | 4 | .. | .. | .. |
| CcJ2NnWw | .. | 8 | .. | .. |
| CcJ2Nnw2 | .. | .. | 4 | .. |
| CcJ2n2W2 | 2 | .. | .. | .. |
| CcJ2n2Ww | .. | 4 | .. | .. |
| CcJ2n2w2 | .. | .. | 2 | .. |
| c2J2-- | 16 | .. | .. | .. |
| +------+------+------+------+
| Total (64) | 34 | 18 | 9 | 3 |
+-----------------+------+------+------+------+
While, if the progeny were all to survive to maturity, we might
expect to get the proportions of white and of white-and-red progeny
called for, yet, since the red color appears in most cases at an age
_after_ the chicks are described, it will be necessary in comparing
experience with calculation to combine the first two classes as
whites. We then find the proportions given in table 60.
TABLE 60.
+------------+-----------------+-----------------------------+
| | | In the actual 85 |
| Color. | In 64, | individuals. |
| | calculated. +---------------+-------------+
| | | Calculated. | Observed. |
+------------+-----------------+---------------+-------------+
| | | | |
| White. | 52 | 69 | 68 |
| Game. | 9 | 12 | 16 |
| Black. | 3 | 4 | 1 |
+------------+-----------------+---------------+-------------+
The proportion of whites agrees closely with expectation. If this
is not the case with the other two classes, the discrepancy must
be attributed in part to insufficient observations and in part to
the difficulties of precise classification in the early stages.
The result is so close, however, as to lend strong support to our
hypothesis as to the gametic constitution of the parents. Nothing is
more striking, and to the unprejudiced mind more convincing, than the
appearance of typically Game-colored birds in the grandchildren of
wholly white parents.
3. SILKIE × BUFF COCHIN.
(Plates 7, 8.)
By hypothesis this cross is between _cJnwx_ and _CjnwX_. The first
generation should give the zygotic formula _CcJjn2w2Xx_, or, more
simply, _CcJjXx_. The formula differs much from that of either
parent, and the progeny themselves are no less remarkable. They have
a washed-out buff color (since they are heterozygous in both _C_
and _X_), and the Jungle pattern shows itself in the black tail and
slightly redder buff of the wing-bar and hackles in the male. Since
the hybrids are heterozygous in respect to 3 pairs of characters,
when segregation occurs each parent produces 8 kinds of gametes, as
follows: _CJX_, _CJx_, _CjX_, _Cjx_, _cJX_, _cJx_, _cjX_, _cjx_. In
F2 the types listed in table 61 may be expected in 64 offspring.
TABLE 61.--_Distribution of colors, theoretic classes.--Probable
frequency in 64 progeny._
+-----------------+------+------+-------+-------+
| Zygotic |White.|Buff. |Buff + | Game. |
| formula. | | |black. | |
+-----------------+------+------+-------+-------+
| C2J2X2 | .. | .. | 1 | .. |
| C2J2Xx | .. | .. | 2 | .. |
| C2J2x2 | .. | .. | .. | 1 |
| C2JjX2 | .. | .. | 2 | .. |
| C2JjXx | .. | .. | 4 | .. |
| C2Jjx2 | .. | .. | .. | 2 |
| C2j2X2 | .. | 1 | .. | .. |
| C2j2Xx | .. | 2 | .. | .. |
| C2j2x2 | 1 | .. | .. | .. |
| CcJ2X2 | .. | .. | 2 | .. |
| CcJ2Xx | .. | .. | 4 | .. |
| CcJ2x2 | .. | .. | .. | 2 |
| CcJjX2 | .. | .. | 4 | .. |
| CcJjXx | .. | .. | 8 | .. |
| CcJjx2 | .. | .. | .. | 4 |
| Ccj2x2 | .. | 2 | .. | .. |
| Ccj2Xx | .. | 4 | .. | .. |
| Ccj2x2 | 2 | .. | .. | .. |
| c2-- | 16 | .. | .. | .. |
| +------+------+-------+-------+
| Total | 19 | 9 | 27 | 9 |
+-----------------+------+------+-------+-------+
The classification here employed can not be used in detail in
comparing observed results with expectation, for the distinction
between buff and buff-and-black appears only in chicks that have
acquired the permanent plumage. Consequently it will be found
necessary to combine these two classes into one and then make the
comparison--as is done in table 62.
TABLE 62.--_Distribution of colors, combined classes._
+-------------------+-----------+--------------------------+
| Color. | In 64, | In the actual |
| |calculated.| 58 individuals. |
| | +--------------+-----------+
| | | Calculated. | Observed. |
+-------------------+-----------+--------------+-----------+
| Buff (and black).| 36 | 33 | 34 |
| White. | 19 | 17 | 17 |
| Game. | 9 | 8 | 7 |
| +-----------+--------------+-----------+
| Total. | 64 | 58 | 58 |
+-------------------+-----------+--------------+-----------+
The correspondence is certainly close. The hypothesis of factors thus
receives additional support and the variability of the offspring in
the second hybrid generation is sufficiently explained.
4. WHITE LEGHORN × BLACK MINORCA.
As we have already seen, the gametic formula of the White Leghorn is
_CJNWx_ and that of the Minorca is _CJNwx_, so that the F1 generation
has the zygotic formula _C2J2N2Wwx2_ or, more simply, _C2J2N2Ww_.
These heterozygotes are white because of the graying factor, but,
as this factor is diluted, some black shows, particularly in the
females. In F2, on account of there being only 1 heterozygous factor,
only 3 kinds of zygotes are formed, _C2J2N2W2_, _C2J2N2Ww_, and
_C2J2N2w2_, in the proportions 1: 2: 1. Since not only offspring
homozygous in _W_, but also all male heterozygotes, are white
and many female heterozygotes are late in revealing any pigment,
it is necessary to consider together individuals homozygous and
heterozygous in _W_. Consequently we may expect 75 per cent of the
offspring to show white or white-black-speckled plumage, and 25 per
cent black or black and white like the young Minorca. Actually, in
154 offspring (pen 633) I obtained 116 white + white-black + blue,
and 38 black with more or less white and including 4 barred, of which
more later. Expectation is 115.5 and 38.5, respectively.
In another experiment I crossed the F1 hybrids on a pure White
Leghorn and got 41 offspring, all white except 1 that showed some
black specks. All results thus accord with hypothesis.
5. WHITE LEGHORN × BUFF COCHIN.
(Plate 9.)
These two races afford the gametic formulæ _CJNWx_ and _CjnwX_,
respectively. The F1 hybrids consequently have the zygotic formula
_C2JjNnWwXx_. Such hybrids are heterozygous in all factors except
_C_. Such complex heterozygotism, combined with the well-known
sex differences in color of heterozygotes, leads to a very great
diversity of the offspring. As a matter of fact I found, as Hurst
did, that the young were sometimes quite white, sometimes white and
buff, and sometimes showed also a little black. Since there are 4
heterozygous characters, there are 256 possible combinations of
them, which reduce to 81 different kinds of combinations. Owing to
the ambiguous nature of the soma in many of the heterozygotes and to
the relatively small number of offspring, it is useless to compare
theoretical and observed distributions of plumage colors in the
somas. Suffice it to say that white, buff, black, and Game-colored
chicks all appeared in the F2 generation, as well as some with a
mixture of colors, as called for by the hypothesis. White, due to the
powerful graying factor, was the commonest color, buff and black were
about equally common, and each about one-third as abundant as white,
while Games, due to the hypostatic J factor, were about one-third as
common as buff. All this, again, is explicable upon our hypothesis
and upon none other so far proposed. In mating the F2 generation
with each other or with the White Leghorn the result must vary with
the gametic output of the hybrid, which is obviously very different
in different cases. A hen, of a light buff color spangled with white
spots and having a black tail, presumably formed gametes _CJnWX_,
_CJnwX_, _CJNWX_, _CJNwX_. Mated with the White Leghorn, _CJNWx_,
she produced 8 pure whites, 4 whites with some black and red, 2
buff and white, and 3 black with trace of white. Expectation in 16
offspring would be about 4 pure whites, 4 white mixed with pigment, 4
buffs with white (and black?), and 4 blacks mixed with other colors.
This is merely an illustration of the way the confused combinations
of colors become intelligible, and even necessary on the factor
hypothesis.
6. BLACK COCHIN × BUFF COCHIN.
(Plate 10.)
The factors involved in this cross seem to be _CINx_ for the
Black Cochin (in which _I_ stands for the Jungle pattern without
any associated color factor) and _CjnX_ for the Buff Cochin, as
before. The F1 generation has the zygotic composition _C2IjNnXx_,
and the females are all black, except for a variable amount of red
on the hackle, and the males are black and red, like Games. The F2
generation is remarkable. Since 3 factors are heterozygous, there are
64 possible combinations and 27 differing ones. In table 63 is given
a list of these different combinations and of the probable associated
somatic colors. The prefixed number indicates the frequency of each
combination.
TABLE 63.
+----------------------------+----------------------------+----------------------------+
| 1 C2I2N2X2 Black. | 2 C2IiN2X2 Black. | 1 C2i2N2X2 Black. |
| 2 C2I2N2Xx Black. | 4 C2IiN2Xx Black. | 2 C2i2N2Xx Black. |
| 1 C2I2N2x2 Black. | 2 C2IiN2x2 Black. | 1 C2i2N2x2 Black. |
| 2 C2I2NnX2 Black and red.| 4 C2IiNnX2 Black and red.| 2 C2i2NnX2 Black and red.|
| 4 C2I2NnXx Black. | 8 C2IiNnXx Black. | 4 C2i2NnXx Black. |
| 2 C2I2Nnx2 Black. | 4 C2IiNnx2 Black. | 2 C2i2Nnx2 Black. |
| 1 C2I2n2X2 Buff. | 2 C2Iin2X2 Buff. | 1 C2i2n2X2 Buff. |
| 2 C2I2n2Xx Buff. | 4 C2Iin2Xx Buff. | 2 C2i2n2Xx Buff. |
| 1 C2I2n2x2 White. | 2 C2Iin2x2 White. | 1 C2i2n2x2 White. |
+-----------------------------+---------------------------+----------------------------+
Uniting the blacks and black-and-reds (since red appears only in one
sex and often not until late in life) we find the following relation
between the calculated and the observed proportions in 86 offspring:
Calculated, black 65, buff 16, white 5; observed, black 61, buff 17,
white 8.
In still another pen (848) the F2 hybrids were mated to a Buff
Cochin. Only 21 chicks were raised. Expectation is, black 10.4, buff
5.2, white 5.2. Actually there were obtained, black 7, buff 10, white
4. Half of the calculated blacks are really heterozygous in both
black and buff; so expectation is a little uncertain, and probably
should be given as something under 10.4. Also, on account of small
numbers, a close agreement is not to be expected; but calculation and
observation are at least of the same order.
CHAPTER XI.
INHERITANCE OF BLUE COLOR, SPANGLING, AND BARRING.
A. BLUE COLOR.
Color-patterns are generalized, like the barring, spangling, and
"blueing"; or localized, like the wing-bar or hackle and saddle
lacing. We have to consider at present the method of inheritance
of the former of these kinds of color patterns. As is well known
(Bateson, Saunders, and Punnett, 1902, 1903), the Blue or Andalusian
fowl is a heterozygote and, as such, produces white gametes and also
black gametes.[10] The "blue" is, indeed, a fine mosaic of white
and black. The barbules of a blue feather are seen to be finely
barred with alternating pigmented and unpigmented zones. The pigment
consists of the ordinary melanic granules of a dark sepia color.
[10] Wright (1902, p. 401) recognizes the variability
of the blues. He advises the breeder of Andalusians that:
"Black and white ones [offspring] can be weeded out at once;
two or three months later birds absolutely too light, or dark
and smoky, can be selected."
My original blues arose (in pen 502) from a White Leghorn hen B
(recognized as heterozygous but of unknown origin), mated to a black
Minorca. These blues are referred to in my 1906 report. They were
both females and were mated (in pen 636) to a white cock (No. 340)
similarly derived. Of 49 offspring, 11, or over 22 per cent, were
black and 78 per cent either pure white (35 per cent of all), white
with black specks (22.5 per cent) or white-and-black mosaic, _i.
e._, blue (20.4 per cent), but the latter were very variable in the
quality of the blue. Let us designate the whitening factor of the
White Leghorn by _W_ (its absence _w_, resulting in black) and the
blueing by _M_ (its absence by _m_). Then, assuming that the blue
females produce germ-cells _MW_, _Mw_, _mW_, _mw_, in equal numbers,
and that the white male produces the same, we may expect in 16 F2
offspring the combinations shown in table 64.
TABLE 64.--_Combinations in zygotes of the second hybrid
generation of the blue strain._
+----------------+----------------+---------------+
| M2W2 1 white. | MmW2 2 white. | m2W2 1 white. |
| M2Ww 2 blue. | MmWw 4 white. | m2Ww 2 white. |
| M2W2 1 black. | Mmw2 2 black. | m2w2 1 black. |
| |
| Totals: White ten-sixteenths; black |
| four-sixteenths; blue two-sixteenths. |
+-------------------------------------------------+
The relation between the calculated and the actual percentages is as
follows:
White + black specks in females: calculated, 62.5; actual, 57.5.
Black: calculated, 25; actual, 22.1.
Blue: calculated, 12.5; actual, 20.4.
That the agreement is not closer must be attributed to the fact
of small numbers and the premature death of many of the chicks,
in consequence of which their adult plumage colors were not fully
revealed. Also, many "blue" chicks produce white adults with black
specks in the plumage.
It is to be observed that this explanation calls for a special mosaic
(blueing) factor, but this mosaic factor brings about a blue plumage
only when the "white" factor is diluted, _i. e._, heterozygous.
In the next generation (pen 733) I mated 2 blues together. This
mating is generally regarded as a unifactorial one (producing gametes
_WM_, _wM_) and to give in every 4 offspring 1 black, 2 blue, and 1
white. I obtained the expected 50 per cent of blues, but always an
excess of blacks and a deficiency of whites (49:35:16, respectively).
This result is doubtless due to the accident that a large proportion
of the chicks were described young, for it appears from my records
that some blues become white when older and some "blacks" are
certainly _blue-blacks_. The deficiency of whites becomes an _excess_
of whites in the adult stage. The whites obtained from the blues are
usually, but not always, splashed with black spots.
B. SPANGLING.
As is well known, hybrids between black fowl and White Leghorns
are usually white with black patches in the females, while their
brothers are mostly entirely white. This "spangled" condition is a
heterozygous one just as truly as the "blue" condition is. When a
splashed hen is mated to her white brother a certain proportion of
the offspring are splashed again, _i. e._, one-half of 50 per cent
or 25 per cent, that being the proportion of heterozygous females.
Actually in 150 offspring 19.4 per cent were splashed and 18.6 per
cent black, while 62 per cent were recorded (largely from unhatched
chicks) as pure white. The splashing reappears in about the expected
proportion of cases. In my pen 633 I take the spangled females to
form gametes _WS_, _Ws_, _wS_, _ws_, while the male seems to form
gametes _Ws_, _ws_; _S_ being the spangling factor. Then [♀
_WS_, _Ws_, _wS_, _ws_] × [♂ _Ws_, _ws_] gives the combinations
shown in table 65.
TABLE 65.--_Combinations in zygotes of the second hybrid
generation of the spangled strain._
+------------------+-----------------+-----------------+------------------+
| Zygotic formulæ. | Male. | Female. | Both sexes. |
+------------------+-----------------+-----------------+------------------+
| W2Ss | White. | Spangled. | |
| W2s2 | White. | White. | |
| 2WwSs | White, spangled.| Spangled. | |
| 2Wws2 | White. | White. | |
| w2Ss | Black. | Black. | |
| w2s2 | Black. | Black. | |
+------------------+-----------------+-----------------+------------------+
| Total patterns in| | | |
| progeny: | | | |
| White. | Five-eighths. | Three-eighths. | Eight-sixteenths.|
| Spangled. | One-eighth. | Three-eighths. | Four-sixteenths. |
| Black. | Two-eighths. | Four-sixteenths.| Do. |
+------------------+-----------------+-----------------+------------------+
This analysis indicates that we should occasionally see a spangled
male, and this expectation is realized. Thus No. 1250 ♂ is an F2
out of White Leghorn A and the Rose-Combed Black Minorca No. 9. He
is white with black spots covering about 10 per cent of the plumage,
and No. 4222 ♂ of similar origin has much black on his chiefly
white plumage. When they are mated to spangled hens of similar origin
with themselves (pen 775), whites, blacks, and spotted, spangled, and
blues occur in the proportions of 1, 17, and 12, respectively. Here
again there is a deficiency of whites in the birds as described, a
deficiency again probably due to immaturity.
Of the mottled condition all degrees are found, from white splashed
with black to black with white spots; also, blue is very common in
the offspring of two mottled birds. The relation of these patterns
is very complex and much time would be required for their complete
analysis, but it seems certain that there is a spangling or mottling
factor, but that, as in canaries, guinea-pigs, and rats, the precise
pattern is not inherited. There are, to be sure, in poultry, so
called _races_ of spangled birds with well-defined patterns, such
as the spangled Polish, spangled Hamburgs, and so forth, but it
is the experience of breeders that they do not reproduce their
patterns closely. The prize-winning birds--those which conform to
the breeder's ideals--are only a small proportion of each family of
offspring. For instance, the Ancona type of plumage, which is black,
each feather tipped with white, has to be carefully sought for in the
progeny of each Ancona pen. The same is true of the Silver Spangled
and Golden Spangled Hamburgs. There is little true spangling in the
first plumage; the darker chicks prove the best; that is, there is
the same tendency to grow whiter with age that I have noted above.
And, finally, only a few birds in any flock are even fairly good show
birds.
C. BARRING.
The presence of bands of black running at intervals across the
otherwise white feather is a condition found in many types of poultry
as well as various wild birds. It has become a fixed character in
the Barred Plymouth Rock, which derived it in turn from the barred
Dominique, whose barring was probably derived from the Cuckoo birds
of England. Barring is also said to result from some crosses between
white and black birds.
In my breedings barred birds have arisen several times:
(1) _White Cochin × Tosa._--This case was referred to in my earlier
report.[11] In the first generation of hybrids all males were barred.
In the second hybrid generation I got 15 chicks that were white or
nearly so, 25 with the Game color, and 16 barred. Remembering that
only the males are barred and that the young heterozygous females are
classed with Games, it appears that the barring is a heterozygous
condition, occurring actually or potentially in about 50 per cent of
the second hybrid generation and that, the whites and some of the
Games are extracted types. This conclusion is confirmed by further
breeding. In pen 663 I bred 2 extracted white hens of Cochin-Tosa
origin to a white cock and got 12 chicks, of which all were white,
except that 3 showed a trace of reddish color. From the extracted
Games bred together I got 36 chicks, all Games. In the case of this
cross, consequently, barring is clearly heterozygous and confined to
the male sex.[12]
[11]: 1906, page 49, figs. 35, 37, 37a.
[12] Goodale, 1909, has shown that in Plymouth Rocks
males may be and females usually are heterozygous in barring.
There is thus a clear difference between the barring of the
Cochin × Tosa hybrid and that of the Plymouth Rock. The
question of the heterozygous nature of the female sex, fully
discussed by Goodale, will be considered by me in another
place. [Note at time of correcting proof.]
(2) _White Leghorn Bantam × Dark Brahma._--This cross was referred to
in my report of 1906. From the table given there it appears that I
got 5 barred fowl in F1 out of a total of 51. In pen 701 I attempted
to see if I could fix this barring. I used the best barred cock of
the F2 generation and the best barred hens of F1 or F2. The result
was as shown in table 66.
TABLE 66.--_Distribution of color in F2 or F2 hybrids of the
barred strain._
[ABBREVIATIONS: W.L. = White Leghorn; Dk.Br. = Dark Brahma.]
+---------------------------------------+---------------------------------+----------------------------+
| Mother. | Father. | Offspring. |
+------+----+-------------+-------------+------+----+-------------+-------+------+------+------+-------+
| | | | | | | | | | | Dark | |
| No. |Gen.| Races. | Color. | No. |Gen.| Races. |Color. |White.|Black.| Brah.|Barred.|
| | | | | | | | | | | | |
+------+----+-------------+-------------+------+----+-------------+-------+------+------+------+-------+
| | | | | | | | | | | | |
| 721 | F1 |W.L. × Dk.Br.|Dark barred | 1898 | F2 |W.L. × Dk.Br.|Barred.| .... | 5 | 7 | 5 |
| 894 | F2 | Do. |Well barred | 1898 | F2 | Do. | Do. | .... | 9 | 3 | [A]10 |
| 965 | F2 | Do. |Medium barred| 1898 | F2 | Do. | Do. | 2 | 16 | 4 | 8 |
| 1335 | F2 | Do. |Dark barred | 1898 | F2 | Do. | Do. | 1 | 14 | 1 | 2 |
| 1772 | F2 | Do. |Poorly barred| 1898 | F2 | Do. | Do. | .... | 4 | 7 | [B]5 |
| 1915 | F2 | Do. |Fairly barred| 1898 | F2 | Do. | Do. | .... | 10 | 4 | 5 |
| 2576 | F2 | Do. | Do. | 1898 | F2 | Do. | Do. | .... | 9 | 11 | 3 |
| | | +------+------+------+-------+
| | | Totals (145) | 3 | 67 | 37 | 38 |
| | | Percentages | 2.1 | 46.2 | 25.5 | 26.2 |
+------+----+-------------------------------------------------------------+------+------+------+-------+
[A] Including 1 blue.
[B] Including 2 blue.
This result suggests the interpretation that one of the parents,
probably the male, contains both heterozygous black and barring,
while the other parent lacks the supermelanic coat and has homozygous
barring. Then of the offspring half will be barred and half will
be black and, consequently (since only the non-black show their
barring), one-fourth will appear barred, one-fourth will appear of
the Dark Brahma type, and half will be pure black or have the pattern
obscured by the supermelanic coat.
(3) _White Leghorn Bantam × Black Cochin._--In still another
experiment (pen 511) I crossed a White Leghorn bantam and a Black
Cochin as described in my report of 1906. Of 24 hybrids that
developed, 10 were white or nearly so, 7 were black, and 7 were
barred black and white. The White Leghorn was heterozygous in white
(half of the offspring being not white) and heterozygous to barring.
In pen 650 the barred birds were mated together with results as given
in table 67.
On the assumption that the zygotic formula of both hens and cocks is
_BbN2Ww_ (compatible with a barred plumage) we get four-sixteenths
of the offspring white, three-sixteenths mottled or barred and
nine-sixteenths black or Game, thus approximating the observed
result; i.e., 21, 16, 47 as compared with 23, 21, 40. The result
supports the hypothesis of a barring factor, B.
TABLE 67.--_Distribution of color in offspring of barred White
Leghorn × Black Cochin hybrids._
+--------------------------------------+--------------------------------------+-------------------+
| Mother. | Father. | Offspring. |
+---+----+---------------------+-------+---+----+---------------------+-------+---+---------+-----+
|No.|Gen.| Races. |Color. |No.|Gen.| Races. |Color. |Wh.|Spangled,|Black|
| | | | | | | | | | barred | or |
| | | | | | | | | |and blue.|Game.|
+---+----+---------------------+-------+---+----+---------------------+-------+---+---------+-----+
|263| F1 |Bl. Coch. × Wh. Legh.|Barred.|265| F2 |Bl. Coch. × Wh. Legh.|Barred.| 8 | 8 | 16 |
|361| F1 | Do. | Do. |265| F2 | Do. | Do. | 7 | 4 | 15 |
|364| F1 | Do. | Do. |265| F2 | Do. | Do. | 8 | 9 | 9 |
| +---+---------+-----+
| Total. |23 | 21 | 40 |
+-----------------------------------------------------------------------------+---+---------+-----+
CHAPTER XII.
GENERAL DISCUSSION.
A. RELATION OF HEREDITY AND ONTOGENY.
In studying heredity our attention must often be focused on the
ontogenesis of the different characters, and we are sometimes
inclined to regard the adult character as the product of the course
of ontogenesis. But this is a superficial way of looking at things;
the determiners of all characters are in the germ-plasm and together
they direct the development of one part after another in orderly
succession; a modernized form of the pre-formation doctrine seems
logically necessary.
What do we know of the processes that take place in bringing the
fertilized egg, freighted with its specific heredity, to its
destination--the adult form? Modern embryological and cytological
studies give us an insight into many of them. First of all, the egg
has a certain organization that foreshadows something of its fate.
Then cell-divisions begin, at first synchronous, but later becoming
accelerated here and retarded there. Eventually (especially among
animals) these cells become arranged into a membrane whose unequal
growth in limited areas produces foldings. The folding of membranes,
their stretching, local thickenings, or thinnings are largely
the result of local inhibitions of water. Sometimes movements of
individual cells occur out of the membranes into and through cavities
or solid yolk-masses, and by the aggregation of such cells massive
organs are sometimes formed. Local absorption of tissues already
established may be effected in later life by such migratory cells.
Membranes once established may form pockets or linear folds, as in
gastrulation and gland formation; they may become perforated; two
membranes may fuse along areas or lines and a perforation may even
occur at the region of fusion. Linear strands or tubules may grow
out, making connections, as nerves do, with distant organs; tubes
may unite to form a network, or split lengthwise. Finally, membranes
and masses undergo vacuolization, or masses may split apart or fuse
together. Thus in the ontogeny that is proceeding under the control
of heredity all is motion and change.
What are the factors that control all these movements--for these are
the true factors of heredity? We do not know much about them, but we
know some things. We know that cell-divisions occur at particular
times and places under the influence of preceding division planes;
but their normal occurrence may be interfered with by an abnormal
chemical condition of the environment.
We have reason for concluding that each developmental process is
a "response"--a reaction of the living, streaming protoplasm to
changing environment. The nature of the response to any stimulus
probably depends on the chemical constitution of the protoplasm--and
this is hereditary. In an important sense heredity is the control of
ontogeny.
The _specific_ characteristics are mostly those that appear late in
ontogeny. The integumentary folds over the nasal bones of the chick
appear on or about the tenth day. At that time it can be ascertained
whether the comb is median, or multiple, or Y-shaped, or
cup-shaped, or consists of 2 papillæ. In the case of the single-comb
the fold is linear and single; in the case of the pea-comb, linear
and triple; in the case of the rose-comb, quintuple or irregularly
wrinkled over the whole area; in the case of the Polish-comb, there
is a pair of "pocket folds." In the single-combed fowl the single
linear fold grows quickly to a great height and very thin, while
in the pea-comb, with its additional pair of wrinkles, the median
element is not so high as in typical single-combed races; in the
pea-comb there is an additional folding stimulus and a reduced growth
stimulus. In the heterozygote both stimuli are weakened; the lateral
folds are usually much reduced--"are hard to make out," as I stated
in 1906 (p. 35); and the factor that determines the continued growth
(elevation) of the fold is weakened, so that the pea-comb--although
"abnormally high" (1906, p. 35, figs. 20 and 21)--is not nearly as
high as the single-comb of the Minorca (1906, fig. 4).
Two results are evident: first, each character in the heterozygous
condition is reduced, and, second, each is much more variable than
in the homozygous condition. Why is the character reduced? If the
reaction to continued growth of the fold is strong in one race and
weak in the other, then in the heterozygote that reaction, whatever
its nature, is reduced. Why is the reduction in the response so
variable? There is a variation in the irritability or other growing
factor of the embryonic material that is destined to form the fold.
Even Minorcas vary in the growth of the comb, and so do the Dark
Brahmas. Let _G_ be a constant element of the growth factor of the
Minorca's comb; then _G + a_ or _G - a_ will indicate its variants.
Let _g_ be the growth factor of the Brahma's comb, and _g + a_ and
_g - a_ its variants. Then the hybrids of these two races may be of
the following types: _Gg_, _Gg + a_, _Gg - a_, _Gg + 2a_, _Gg - 2a_.
This gives 5 varying conditions instead of 3 and greater extremes of
variation.
In the foregoing case I have assumed that the positive character is
that of increased growth in the Minorca; but the positive character
may be an _inhibition_ to indefinite growth of the pea-comb.
Heredity may be conceived of as exerting at all points a control on
developmental processes--sometimes initiating and continuing this;
but often, on the other hand, slowing down or wholly inhibiting
that. The inhibition of a process is quite as _positive_ a function
of heredity as its initiation. The hair of a young rabbit grows
until it attains a certain length and then the growth ceases. The
growing character is a youthful, embryonic one; the new character is
the stoppage of growth. Similarly the young feathers of birds grow
continuously until something intervenes that stops the growth and
dries up the sheath. Now, in Angora rabbits and long-tailed fowl
the epidermal organ continues its embryonic growth indefinitely;
the something that intervenes to stop growth is absent. There is no
reason for regarding the long hair or long feather as a positive
condition and short hair or feather as due to its absence.
Again, Mediterranean fowl have non-feathered shanks; but in Asiatics
the feet are feathered like the rest of the body (except the soles
and face). It has been assumed that _boot_ is an additional character
and should be dominant over absence of boot. But, on the other hand,
we may well think of the capacity of producing feathers as general to
the skin. From this point of view the real question is, what prevents
feather production on the eyelids, comb, wattles, and shank? It
seems equally probable that there is an inhibitor of feather-growth
for these few areas as that every conceivable area of the body has
its special stimulus factor for feather development; or even as
that there is such a factor to each separate feather-tract. In the
Minorca, then, the inhibitor of boot is present; in the Silkie a weak
heterozygous inhibition appears; but in the Dark Brahma there is no
inhibitor and feathers extend down from the heel over the whole of
front and sides of the foot and even on the upper surface of the
toes--just as they do over the anterior appendages.
The case of the rumpless fowl is important in relation to the
hypothesis of inhibitors. Either tail-production depends on a special
factor _TT_, which is diluted, as _Tt_, in the heterozygote; or else
there is a tail inhibitor, _II_, which is diluted, as _Ii_, in the
heterozygote. In F2 we expect, on the one hypothesis, 25 per cent
_tt_, giving no tail, and 25 per cent _TT_, giving tail; on the other
hypothesis 25 per cent _ii_, giving tail, and 25 per cent _II_,
giving no tail. Actually we get all tailed in some cases; in others
25 per cent with no tail. Which hypothesis best fits the facts?
Which is the more probable--that the 25 per cent recessive no-tail
should produce a tail (as it were, out of nothing) or that the 25
per cent dominant tail inhibitor should be ineffective, permitting
the development of a tail? It is clear that the ontogenetic failure
of an inhibitor is easier to understand than the development of a
character that is not represented at all in the germ-plasm. This
matter is treated in another connection in the next section. But
the present point is that it is equally in accord with the facts to
regard heredity as initiating and inhibiting processes. If, indeed,
processes were not regularly inhibited, they must, when once started,
go on indefinitely, as do the hairs of Angora goats and wonder-horses.
As we have seen, ontogeny is not completed at hatching or birth. Many
characters are at that time undeveloped. Hence, not infrequently
the recessive condition is at first seen and is only later
replaced by the dominant condition. The reverse sequence will
rarely be followed, because development rarely, except in cases of
degeneration, moves backward. One of the familiar cases of this sort
is human hair-color. In youth this is frequently flaxen, later it
becomes light brown, and eventually it may become dark brown. Darwin
gives a number of examples in his Chapter XII of Animals and Plants
under Domestication. To these I may add some from my own experience.
The hybrids between white and gray Java sparrows are at first light
and later become of a slaty gray like the dark parent. Many black
fowl gain white feathers as they grow older, and every fancier knows
that birds with complex white-and-black patterns can usually be
"exhibited" only once, on account of loss of "standard" coloration
late in life. In these cases the advanced condition in the series
of melanic colors appears only late in ontogeny.[13] Similarly Lang
(1908, p. 54) finds that in snail hybrids often the young shells
have the recessive yellow color, only later in life showing the
dominant red color. This is, of course, no reversal of dominance in
ontogeny, but mere ontogenesis of pigmentation. So in general, since
the recessive condition is absence of the character or its low stage
of development and the dominant condition is presence of the full
character, the individual in ontogenesis may exhibit in succession
the recessive and then the dominant character, but not in the reverse
order.
[13]: Does the graying of human hair represent an
ontogenetically advanced condition of the melanic pigment as
yellow represents the embryonic condition?
B. DOMINANCE AND RECESSIVENESS.
If segregation is the cornerstone of modern studies in heredity,
dominance forms an important part, at least, of the foundation. In
any case, a critical examination of dominance is now required; the
more so since its significance and value have often been doubted.
First, how is a dominant character to be defined? It has been defined
both on the basis of visible results in mating and on the basis of
its essential nature. On the basis of visible results in hybridizing
dominant characters may be defined as Mendel (1866, p. 11) defined
them: "jene Merkmale, welche ganz oder fast unverändert in die
Hybride-Verbindung übergehen." Bateson's translation (1902, p. 49)
renders this passage: "those characters which are transmitted entire,
or almost unchanged in the hybridization."
On the basis of the essential nature of the dominant character there
has obtained a great diversity of definitions. Thus de Vries (1900,
p. 85) suggested that the "systematically higher" character is the
dominating one, and, again (1902, pp. 33, 145), that the dominant
character is the phylo-genetically older one. Many have suggested
that it is the positive or present character that dominates over the
negative, latent or absent. This last idea has become the prevailing
one and its history is worth summarizing.
As early as 1902, Correns used as Mendelian pairs, presence of
coloring material and absence; also modification into yellow and _no_
modification. In 1905, he extended somewhat this use of present and
absent characters, _k_ (keine) preceding the symbol of a character as
a negative. Still he did not pretend to generalize the relation of
dominance and recessiveness to be that of presence and absence. In
1903 (p. 146) de Vries stated that in very many cases Mendel's law
held when one quality is active and the other latent, and that the
active quality is dominant. His illustrations show that by activity
he meant essentially presence, by latency absence from the visible
soma. Bateson's third report (1906) applies presence and absence
to several additional cases, and, at the International Genetics
Conference of that year, Hurst developed the presence-and-absence
hypothesis, favoring the view that the factor for absence is nothing
at all, but finding that certain cases, such as Angora coat, offer a
difficulty. At the same meeting I suggested that "a variation * * *
that is due to abbreviation of the ontogenetic process, which depends
on something having dropped out, will be recessive," a progressive
variation dominant; and in 1908 I expressed the conclusion that
"dominance in heredity appears when a stronger determiner meets a
weaker determiner in the germ. The extreme case is that in which a
strong determiner meets a determiner so weak as to be practically
absent, as when a red flower is crossed with white." I suggested that
in some cases of recessiveness of an apparent advanced condition,
like Angora hair, the dominant factor is an inhibitor. In the
last year or two the presence-and-absence theory has gained wide
acceptance, but I still think the cases where there is dominance of
the advanced condition over the less advanced--of the quantitatively
well-developed over the quantitatively less well-developed--have
not been sufficiently considered. In human hair-color any other
hypothesis demands that there are many units in the higher grades
of pigmentation and fewer in the lower grades and that the presence
of the surplus factor in any other higher grade dominates over
its absence in the next lower grade; but there is no evidence in
human hair-color of distinct, discontinuous units in the common
yellow-brown series. And, in ontogeny, the different grades of color
form a _continuous_ series whose development proceeds throughout
early life and may even be stimulated to an advanced stage of
darkening by disease. The _cessation_ of color development may take
place at any point, and this seems incompatible with the theory of
unit-characters for the different grades of human hair-color. In
the present paper, on the other hand, the characters dealt with are
mostly unit-characters and their quantitative variations mostly
heterozygotic. Even the case of the Silkie boot (table 31, C)
referred to in an earlier paper[14] as illustrating recessiveness
of the less advanced condition proves, on further analysis, to be
a case of heterozygotism. It seems highly probable that the future
will show that many more advanced or progressive conditions are
really due to one or more unit-characters not present in the less
advanced condition. In that case it will appear that there is perfect
accord in the two statements that the progressive condition and the
"present" factor are dominant.
[14]: Davenport, 1908, page 60.
The definition of dominance on the ground of results meets at the
outset with a difficulty the germ of which is observable in Mendel's
cautious statement "ganz oder _fast_ unverändert." Even Mendel
observed that the hybrids between white-flowered and purple-red
flowered peas have flowers less intensely colored than the darker
parent. The experiments of the last seven years have shown that
the "dominant" character is often very greatly changed--indeed,
in extreme cases a blending of characters may occur--in the first
generation. Correns (1900 _b_, p. 110) very early stated that in a
certain set of crosses between good species the hybrids showed the
character of _both_ parents, only reduced, but in varying degrees.
Bateson and Saunders (1902, p. 23) found in crossing two forms of
_Datura_ that--
Although the offspring resulting from a cross between any two
of the forms employed are usually indistinguishable from the
type which is dominant as regards the particular character
crossed, yet in other cases the intensity of a dominant
character may be more or less diminished either in particular
individuals or in particular parts of one individual. In
_Tatula-Stramonium_ cross-breds the corolla is often paler in
color than that of the dominant parent (as has already been
noticed by Naudin), but even in the palest specimens the deep
blue color of the unopened anthers leaves no doubt as to the
presence of the dominant color element. * * * The occurrence
of intermediate forms was also occasionally noticeable in the
fruits. Among the large number of capsules examined, there
were some of the mosaic type, in which part of the capsule was
prickly and the remainder smooth, while others, suggesting a
blend, were more or less prickly all over, but the prickles
were much reduced in size, and often formed mere tubercles.
Bateson and Saunders further showed (1902, p. 123) that in the case
of comb and extra-toe in poultry "the cross-bred may show some
blending and * * * the intensity of the dominant character is often
considerably reduced."
Correns (1905, p. 9) pointed out that there was known, even at
that time, a complete series of cases at one extreme of which one
determiner completely hindered the appearance of the other, while
at the opposite end of the series the hybrid showed an intermediate
condition, both determiners appearing with equal strength.
The following year, in my first report on Inheritance in Poultry,
I laid great stress on the imperfection of dominance, and this
phenomenon has become more striking and clear in the subsequent
years, until in the present paper it is recognized as the key to the
explanation of many apparently anomalous types of heredity.
The first case in the present work in which imperfection of dominance
is considered is that of the hybrids between I and oo comb.
Here median comb is mated with no-median. Each somatic cell of
the hybrid--at least in the comb region--has only half the full
determiner for median comb. The determiner is weakened, and so the
median comb is imperfectly developed, namely, at the anterior end of
its proper territory. The weakening varies much in degree in the
heterozygote. The median comb may be reduced to 70 per cent of its
normal length or it may not develop at all.
The second case of imperfection of dominance is that of
polydactylism. Extra-toe mated to normal gives extra-toe in 73 per
cent only of the offspring in the case of the Houdans. Any trace of 6
toes (on one or both feet) is found in only 12 per cent of the hybrid
offspring from a 6-toed Silkie parent. Certainly dominance here is
very like blending.
The third case of imperfection of dominance is that of syndactylism.
No syndactyls were noticed in F1. My first conclusion was that
syndactylism is recessive; but later studies have shown that it is
dominant and that all matings of two syndactyl parents yield about 56
per cent syndactyl offspring.
Rumplessness gives an illustration of how dominance may be so weak
as to be absent altogether; so that from F1 alone the erroneous
conclusion is drawn that it is recessive; indeed, in one strain, only
faint traces of the character made their appearance in successive
generations.
Finally, winglessness is a character which _appears_ not to be
inherited at all. Nevertheless our experience with rumplessness
leads us to suspect that winglessness also is an impotently dominant
character.
Looking at the matter frankly and without prejudice, the question
must be answered: Has not the whole hypothesis of dominance become
_reductio ad absurdum_? What visible criterion of dominance remains,
where dominance fails completely? All the usual statistical landmarks
of proportional appearance in successive generations being lost, can
one properly speak of dominance and recessiveness at all?
Amid the general ruin of criteria, however, one means of detecting
dominance remains. That extracted character which in F2 or subsequent
generations shows in homologous[15] matings in some families a wide
range of variability is dominant, while that extracted character
which constantly, in all homologous matings, shows no or very little
variation is recessive.
[15] By homologous matings I mean those in which the
germ-plasms of _both_ parents are in the same condition with
reference to the unit-character; _i. e._, both either possess
it pure or lack it altogether.
The reason for this difference in the inheritableness of the two
conditions is easy to understand on the principles enumerated in
the last section. A positive character has a real ontogeny. But, as
we have seen, the development of any character may be _interrupted_
at any stage. Most aberrations among organisms are due to a
retardation or failure of normal development. In human affairs we
recognize this tendency in the terms "degenerates" and "defectives"
(constituting from 2 to 4 per cent of the population). Indeed, there
are few persons who are not defective in some physical or psychical
character. In cases where the commonest form of abnormality is due to
a development _in excess_ it seems probable that a normal restraining
or inhibiting factor is defective or absent. On page 88 I tried to
show how common in ontogeny such restraining and inhibiting factors
are. Since ontogenetic processes are so often cut short by external
conditions, we can understand the _variability_ in the degree of
development of positive characters.
On the other hand, whenever the fundamental hereditary stimulus or
the material for a character is absent from the germ-plasm of both
parents, then it can appear in none of the offspring; they will
be practically invariable in respect to this condition. Only the
ontogenetic fluctuations of other real characters may influence
the defect. Consequently the absent state reproduces itself, the
"recessive breeds true."
The considerations here presented bear upon the hypothesis of change
of dominance. Bateson and Punnett (1905, p. 114) say of poultry: "The
normal foot, though commonly recessive, may sometimes _dominate_ the
extra-toe character." This idea of occasional _change_ in dominance
has been expressed more than once in the literature. I think the
phrase an unfortunate one. In my earlier report[16] I urged that a
characteristic that is anywhere dominant is so without regard to race
or species involved. If this is so it is clearly improbable that it
should vary from individual to individual, or in the same individual
at different times. Rather in view of the imperfection of dominance
we should say that a dominant character sometimes fails to develop,
in which case it is absent from the progeny; that is all. It is
particularly apt to fail of development when dilute--_heterozygous_.
[16] Davenport, 1906, page 86.
C. POTENCY.
Perhaps an apology is needed for introducing the much-abused word
"potency"; but there is hardly another that can be so readily adapted
to the precise definition I desire to give to it. The potency of a
character may be defined as the capacity of its germinal determiner
to complete its entire ontogeny. If we think of every character as
being represented in the germ by a determiner, then we must recognize
the fact that this determiner may sometimes develop fully, sometimes
imperfectly, and sometimes not at all. When such a failure occurs in
a normal strain a sport results.
Potency is variable. Even in a pure strain a determiner does not
always develop fully, and this is an important cause of individual
variability. But in a heterozygote potency is usually more or less
reduced. When the reduction is slight _dominance_ is nearly complete;
but when the reduction is great dominance is more or less incomplete
and, in the extreme case, may be absent altogether. The series of
cases of varying perfection of dominance described in this work
illustrate at the same time varying potency. The extreme case is that
of the rumpless fowl. The character in this case is an inhibitor
of tail development. This character has arisen among vertebrates
repeatedly and has become perpetuated in some amphibia and primates,
including man. In the case of our cock No. 117, the action of the
inhibitor is very weak, so that in the heterozygote the development
of the tail is not interfered with at all and even in extracted
dominants it interferes little with tail development, so that it
makes itself felt only in reduced size of the uropygium and in bent
or shortened back. But in No. 116 the inhibiting determiner is
strong. It develops fully in about 47 per cent of the heterozygotes
and 2 extracted dominants may produce a family in _all_ of which
the tail's development is inhibited. In the case of the rumpless
condition that arose apparently _de novo_ in my yards, the new
inhibitor showed an intermediate potency completely stopping the tail
development in 1 out of 25 heterozygotes. These three cases afford
a striking illustration of a variation in the potency of the same
inhibiting character in different strains.
Not only is potency variable, but its variations seem, in some cases,
to be inheritable. This we have seen to be the case with the Y-comb
(p. 15); with the extra-toed condition of Houdans (p. 23); and with
rumplessness (_cf._ offspring of No. 117 as compared with No. 116,
p. 40). On the other hand, the extra-toed condition of Silkies, the
grade of clean shank, and the degree of closure of nostril seem not
to be inherited.
D. REVERSION AND THE FACTOR HYPOTHESIS.
The brilliant development of the factor hypothesis, only dimly
fore-shadowed by Mendel[17] (1866, p. 38), clearly expressed by
Correns (1892), applied to animals by Cuénot, and further elaborated
by Bateson and Castle and their pupils, has quite changed the
methods of work in heredity. More forcibly than ever is it brought
home to us that the constitution of the germ-plasm--not merely
the somatic character--is the object of our investigation. With
this principle fully grasped the existence of cryptomeres and the
resolution of characters have become clearer. But the most striking
result accomplished has been that of clearing up the whole range of
phenomena formerly placed in the category of "reversion." No idea
without a semblance of inductive explanation has been more generally
accepted in the Darwinian sense both by professed biologists and
practical breeders than this. Not only was the fact of recurrence
of ancestral types in domesticated organisms accepted, but the idea
that, in some way, hybridization _per se_ destroyed the results
of breeding under domestication was maintained.[18] Now we know
that, under domestication, many races have been preserved that are
characterized by a deficiency of a character or by a new, additional
one, and that hybridization, by bringing together again those
characters that are found in the ancestral species, may bring about
again individuals of the ancestral type. There is nothing more
mysterious about reversion, from the modern standpoint, than about
forming a word from the proper combination of letters.
[17] Mendel's expression on this subject is translated
by Bateson (1902, p. 84) as follows: "Whoever studies the
coloration which results in ornamental plants from similar
fertilization can hardly escape the conviction that here also
the development follows a definite law which possibly finds its
expression _in the combination of several independent color
characters_. (The italics are Mendel's.)
[18] "An inherent tendency to reversion is evolved
through some disturbance in the organization caused by the act
of crossing." (Darwin, Animals and Plants under Domestication,
Chapter XIII, section, "Summary on proximate causes leading to
reversion.")
E. THE LIMITS OF SELECTION.
In the last few decades the view has been widespread that characters
can be built up from perhaps nothing at all by selecting in each
generation the merely quantitative variation that goes farthest in
the desired direction. I have made two tests of this view, using the
plumage color of poultry.
(1) _Increasing the red in the Dark Brahma × Minorca cross._--The
Dark Brahma[19] belongs to the group of poultry that contains a
majority of characters derived from the Aseel type. Nevertheless, its
plumage is closely related to that of the Jungle-fowl, from which it
may be derived on the assumption that the red part of the pattern has
become, for the most part, white. However, a little red remains on
the middle of the upper feathers of the wing-bar. I crossed such a
bird with a Black Minorca, and, as reported in my earlier work,[20]
the offspring were all black, except that the males showed some red
on the wing-bar. The amount of red varied in the different males,
and I decided to test the possibility of much increasing the amount
of the red by selection in successive generations. So I chose the
reddest cock to head the pen. In this pen (No. 632) 222 chicks were
produced and grew to a stage in which their adult color could be
determined. Of these 222 chicks, 160, or 72 per cent, were black,
without red; 24, or 10.8 per cent, were black with some red; 38, or
11.7 per cent, were typical Dark Brahmas, and 9 others, or 4.5 per
cent, were modified Dark Brahmas.
[19] Plate 11.
[20] Davenport, 1906, page 35.
The following year (pen 732) I bred a cock derived from the last
year's pen, a bird that resembled much the male Dark Brahma (except
that it was somewhat darker), to sundry hens, hybrids between the
Dark Brahma and Minorca--some of the first and some of a later hybrid
generation, but all black except that some of the 1906 birds had a
little buff on the breast and the primaries. The F1 (black) × F2
(Dark Brahma) gave 51 per cent black offspring, 27 per cent with a
black-and-red Game pattern, and 22 per cent with the Dark Brahma
pattern devoid of red. Thus the third generation suddenly gave me a
red-and-black Game-colored bird (plate 12)!
My interpretation of the foregoing results is as follows: The Dark
Brahma gametic formula proves to be _CIrnwx_, whereas the Black
Minorca is _C(IR)Nwx_, where (_IR_) is equivalent to, and merely
a further analysis of, the _J_ of the formula of the Minorca as
given in earlier sections. The _I_ stands for the Jungle pattern
without red and _R_ is the red element in that pattern. Obviously
_N_ and _R_ are the differential factors, 4 kinds of gametes occur
in F1, and in every 16 offspring these factors are combined in the
following proportions: 9 _NR_, 3 _Nr_, 3 _nR_, 1 _nr_ (compare the
distribution of color types in the 222 offspring of pen 632). The F2
male selected as father of the next generation (in pen 732) was an
extracted Dark Brahma in coloration and probably formed only 1 kind
of gamete, _nr_; but the hens were heterozygous in respect to _N_
and _R_. Consequently 4 kinds of zygotes are to be expected in F3;
and expectation was realized as indicated in table 68.
TABLE 68.
+-------------+------------+------------+-----------+-----------+
| | _NnRr._ | _Nnr2._ | _n2Rr._ | _n2r2._ |
| | | | | |
| | Black with | Black. | Game. | Dark |
| | traces of | | | Brahma |
| | red in | | | (without |
| | male. | | | red). |
+-------------+------------+------------+-----------+-----------+
| | _P. ct._ | _P. ct._ | _P. ct._ |
| | | | |
| Expectation | 50 | 25 | 25 |
| Realization | 51 | 27 | 22 |
+-------------+-------------------------+-----------+-----------+
In the case where both parents are F2 or F3 it is impossible to
summate results, since the gametic formulæ of the different parents
are so diverse; but the same types of solid blacks, black with trace
of red in the males, Game-colored males and females, and Game with
red replaced by white repeatedly occur. My plan of increasing red in
the Dark Brahmas met with wholly unexpectedly prompt success, but not
in the way anticipated. The result was not due to selection, but to
the recombination of the factors necessary to make the Game plumage
coloration.
(2) _Production of a buff race by selection._--The second test was
directed toward the production _de novo_ of a new buff race from a
Game fowl.
As is well known, all of our red and "buff" races, like the Buff
Leghorn, Rhode Island Red, and others, have been derived from the
Buff Cochin that came to us from China. The fact that a buff bird
has, so far as I have been able to learn, not been produced in
western countries indicates the probability that it can not be so
produced at will; but the attempt seemed worth while.
I began with a Black Breasted Red Game because its plumage color
is that of the primitive ancestor of domesticated poultry, and on
that hypothesis the ancestor of the buff races. If these buff races
were produced by extending the red through selection of the reddest
offspring, that should be possible now as in the past.
A start in the direction of creating a buff bird would seem to
require the elimination of the black. By crossing a black and red
Game with a White Leghorn I got, in 1905, 2 white pullets with red on
breast and some black specks. By crossing a Game Bantam (wingless)
with a White Leghorn I got white birds with red present on wing-bar
of male and breast of females and also some black spots.
In 1906 I mated 2 of these white (+ red) bantam hybrid hens with a
hybrid cock and obtained again red on the wing-coverts of some white
hybrids, while some were without red. From one of the hens I got 4
offspring, or 20 per cent of all, with buff on hackle-lacing, breast,
and wing-coverts.
In 1907 I mated a prevailingly white male of the preceding year,
that had red wing-bar, hackle, and breast, with the reddest females
and obtained, along with pure whites and blacks and barred birds,
these colors combined with red in various degrees, but not clearly
in advance of the reddest of 1906. In 1908 I mated a white male,
having red as in the Game, with my reddest hybrids. Again, white and
white-and-buff birds appeared, but they showed no advance, except in
one instance, among 138 young. This individual (No. 7950), derived
exclusively from the Black-red Game and White Leghorn on one side and
on the other from the White Leghorn-Game Bantam cross, had a _uniform
buff_ down. Unfortunately the chick quickly died.
The conclusion is that after three years of selection of the reddest
offspring no appreciable increase of the red was observed--except for
the remarkable case of one undeveloped chick with completely buff
down. This, indeed, looks like a sport, or, perhaps, it is due to
unsuspected factors. The experiment will be continued.
F. NON-INHERITABLE CHARACTERS.
So well-nigh universal is heredity that it is justifiable to
entertain a doubt whether any character may fail of inheritance. So
far as my experience goes, non-inheritable characters are such as
are weak in ontogeny, so that they may readily fail of development
even when conditions are propitious; or else they are so complex--so
far removed from simple unit-characters--that their heritability
in accordance with established canons is obscured. The first case
is apparently illustrated by the rumpless cock (No. 117) and the
wingless fowl; the second case by lop-comb and by right-and-left
alternatives in general.
Apart from the distinct _characters_ that fall under these two
categories there are the fluctuating quantitative _conditions_. These
depend for the most part, as already pointed out, on variations in
the point at which the ontogeny of a character is stopped; and the
stopping-point is, in turn, often, if not usually, determined by
external conditions which favor or restrict the ontogeny. Whether
or not such quantitative variations are transmitted is still
doubtful. Our experiment in increasing qualities, such as redness in
plumage-color, by selection of quantitative fluctuations have not
been successful in the sense anticipated; neither have selections of
comb, polydactylism, or syndactylism. Recently, prolonged attempts
at the Maine Agricultural Experiment Station to increase egg-yield
of poultry by selection have been without result. Apparently,
within limits, these quantitative variations have so exclusively an
ontogenetic signification that they are not reproduced so long, at
least, as environmental conditions are not allowed to vary widely.
The conclusions which others have reached, and upon which de Vries
has laid the greatest stress, that quantitative and qualitative
characters differ fundamentally in their heritability is supported by
our experiments.
G. THE RÔLE OF HYBRIDIZATION IN EVOLUTION.
The criticism has often been made of modern studies in hybridization
that they are really unimportant for evolution because hybridization
is uncommon in nature. Even at the beginning of the new era it could
be replied that, first, we did not know how common hybridization
might turn out to be in nature, and, second, that certainly in human
marriage and among domesticated animals and plants, intermixing of
characters played a most important part, and, finally, the laws of
inheritance of characters were of such grave physiological import
as to deserve study wholly apart from any question of the rôle of
hybridization in evolution.
The last decade of work has made clear many things that were before
uncertain. We now realize that in nature hybridization may and
actually does proceed extensively. Dr. Ezra Brainerd has shown how
many wild "species" of _Viola_ have arisen by hybridization, as may
be proved by extracting from them combinations of characters that
are found in the species that are undoubtedly ancestral to them.
In such highly variable animals as _Helix nemoralis_ and _Helix
hortensis_ it is very probable that individuals with dissimilar
characters regularly mate in nature and transmit diverse combinations
of characters to their progeny. Indeed, if one examines a table of
species of a genus or of varieties of a species one is struck by
the paucity of distinctive characters. The way in which species, as
found in nature, are made up of different combinations of the same
characters is illustrated by the following example, taken almost at
random. Among the earwigs is the genus _Opisthocosmia_, of which the
5 species known from Sumatra alone may be considered. They differ,
among other qualities, chiefly in the following characters (Bormans
and Kraus, 1900):
Size: _A_, large; _a_, small.
Wing-scale: _B_, brown; _b_, yellow.
Antennal joints: _C_, unlike in color; _c_, uniform.
Forceps at base: _D_, separated; _d_, not separated.
Edge of forceps: _E_, toothed; _e_, not toothed.
Fourth and fifth abdominal segments: _F_, granular; _f_, not granular.
The combinations of these characters that are found are as follows:
_Opisthocosmia ornata_: _AbcDEF_.
_insignis_: _ABcDEf_.
_longipes_: _AbCDEf_.
_tenella_: _AbCdef_.
_minuscula_: _aBCDEf_.
Other species occur, in other countries, showing a different
combination of characters, and there are characters not contained in
this list, which is purposely reduced to a simple form; but the same
principles apply generally.
The bearing upon evolution of the fact that species are varying
combinations of relatively few characters is most important. Combined
with the fact of hybridization it indicates that the main problem
of evolution is that of the origin of specific characteristics.
A character, once arisen in an individual, may become a part of
any species with which that individual can hybridize. Given the
successive origin of the characters _A_, _B_, _C_, _D_, _E_, _F_,
in various individuals capable of intergenerating with the mass of
the species, it is clear that such characters would in time become
similarly combined on many individuals; and the similar individuals,
taken together, would constitute a new species. The adjustment of the
species would be perfected by the elimination of such combinations as
were disadvantageous.
COLD SPRING HARBOR, NEW YORK,
_May 20, 1909_.
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[Illustration: PLATE 1
Jungle Fowl, male, showing distribution of black and red elements
of pattern.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 2
Jungle Fowl, female, showing coloration and pattern.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 3
White-faced Black Spanish, male.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 4
First generation hybrid between White-face Black Spanish Cock and
White Silkie Hen.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 5
First generation hybrid between Black Minorca Cock and White Silkie
Hen.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 6
Second hybrid generation between Silkie and Spanish Minorca, (No.
3898) female.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 7
Buff Cochin, (No. 545) male.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 8
Cock of first hybrid generation between Black Cochin and Buff
Cochin.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 9
Cockerel (No. 6094) of first hybrid generation between Buff Cochin
Cock and Silkie Hen.
A. Hoen & Co. Baltimore.]
[Illustration: PLATE 10
Cockerel (No. 2561) of second hybrid generation between Buff Cochin
and White Leghorn.
A. Hoen & Co. Baltimore. Kako Morita, pinx.]
[Illustration: PLATE 11
Dark Brahma, (No. 122) male.
The detailed feathers are in order from right to left from first,
third and fourth wing coverts.
A. Hoen & Co. Baltimore. Kenji Toda, pinx.]
[Illustration: PLATE 12
A cock (No. 5257) of the third hybrid generation between a
single-comb Black Minorca and a Dark Brahma shown in plate 6.
The detailed feathers are in order from right to left from the
first, second, fourth and third wing coverts.
A. Hoen & Co. Baltimore. Kenji Toda, pinx.]
Transcriber's notes.
Words in italics are surrounded by _underscored-.
Clear printer's errors were corrected, but original spelling was not
modified or harmonized.
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