INHERITANCE 



299 



with-blue birds. So again it is clear that segregation is involved, as 

 it is in innumerable instances where no sharp distinction can be 

 made between complete and incomplete dominance. (Fig. 187.) 



Illustrations of some of the complications are afforded by cases 

 of blending inheritance that result from the cumulative action of 



Xjchard EdevHarrljon? 



Fig. 187. — Cross of white-splashed- with-blue and black fowls, giving in 

 the Fi all blue Andalusians, and in the F 2 one white-splashed-with-blue to two 

 blue Andalusians to one black. 



several pairs of genes (multiple genes) as in the cross of white and 

 black human races. The mulatto (Fi), from a cross between an 

 individual homozygous for white and an individual homozygous 

 for black, is intermediate in skin color between the parental types, 

 and in the F 2 and later generations gives a series of gradations 

 between white and black but rarely pure white or black offspring. 

 Assuming that three pairs of genes for color are involved, the 

 genetic constitution of the black race may be represented by 

 AABBCC and that of the white race by aabbcc. Accordingly the 

 hybrid, or mulatto, has the genetic constitution AaBbCc and is 

 intermediate in color since only half of the genes for black pig- 



