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.