No. 588] 



F t BLEND AND GEN1C PURITY 



745 



a single gene — from the paternal or the maternal line only 

 —for such trait, in the zygote, is not sufficient to give a 

 somatic development of the trait equal to that possessed 

 by the duplex parent. In such cases, therefore, the unit 

 trait in question is blended in the F x soma — a case of 

 imperfection of dominance. 2 Nevertheless, in such cases 

 segregation is just as clean-cut in the germ-plasm as it is 

 in the cases accompanied by strong somatic dominance. 



In Andalusian fowl "W" — dominant splashed-white — 

 and "N" — (nigrum) black— are two opposing and allelo- 

 morphic genes of nearly equal valence in ontogenesis. 

 Their combination and interaction determine plumage- 

 color in the offspring. The black Andalusian is duplex 

 for black plumage-pigment, while the splashed-white is 

 duplex for dominant splashed-white. The F 1 offspring 

 are " blue" — a shade really intermediate between the 

 white and the black. Moreover, the genes "W" and " N " 

 evidently lie in the same 3 chromosome. The evidence for 

 this consists in the fact that in the F 2 generation, result- 

 ing from inbreeding two blue Andalusians, neither albinic 

 white nor jungle 4 — pure or modified — patterned fowl re- 

 sult, which would be the case if "N" and "W" lay in 

 different chromosomes, permitting, in some F 2 zygotic 

 combinations, the elimination of both "N" and "W." 

 For further explanation of this particular type of blended 

 inheritance see the accompanying figure descriptive of the 

 mechanical chart " Plumage-Color in Andalusian Fowl." 



The second type— that of multiple factors— is typified 

 by the inheritance of black skin-pigment in man. It is a 

 matter of common knowledge that a mulatto of the first 

 generation is about intermediate in density of black skin- 

 pigment between his white and his black parents. In 1913 



2 " Imperfection of Dominance," American Breeders Magazine, No. 1, 

 Vol. 1, p. 39, 1910, by C. B. Davenport. 



a "Heredity and Sex," p. 93 et seq. (Columbia University Press, 1913), 

 by Thomas H. Morgan. 



4 "New Views about Eeversion," Proceedings of the American Philo- 

 sophical Society, Vol. XLIX, No. 196, 1910, by C. B. Davenport. 



