Inheritance of Characteristics in Domestic Fowl

CHARLES B. DAVENPORT, Director of the Station for Experimental Evolution, Carnegie Institution of Washington.
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


INHERITANCE OF CHARACTERISTICS IN DOMESTIC FOWL. BY CHARLES B. DAVENPORT.
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.
Syndactylism illustrates another step in the series of decreasing potency of the dominant. On not one of the F 1 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.