Inheritance of Characteristics in Domestic Fowl

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

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INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL.

CHARLES B. DAVENPORT.

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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.

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.

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