HEREDITY 



a male, since it lacks X; the second type will 

 contain B X, a bird heterozygous in barring, 

 and a female, since it contains X. This agrees 

 with the experimental result (see Fig. 52). 



A heterozygous barred male will form two 

 kinds of sperm, only one of which will contain 

 B, If such a male be mated with a barred 

 female, four sorts of zygotes should result, as 

 follows : 



Gametes of heterozygous barred male = B and , 

 Gametes of barred female = B and X, 



Zygotes = BB (homozygous barred male); B 



(heterozygous barred male), B'X (barred female), and 



X (non-barred female). 



The observed result of this cross accords 

 fully with the foregoing expectation. 



The sex-limited inheritance of barring in 

 fowls may be explained, as we have just seen, 

 on the assumption that the female is the hetero- 

 zygous sex. The same is true of sex-limited 

 inheritance in canary-birds and in the moth, 

 Abraxas, according to Bateson and Doncaster. 

 But these relations are exactly reversed in the 

 pomace-fly, Drosophila ampelophila, according 

 to Morgan. 



174 



