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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, 