Heredity and Sex 



75 



MALE 



female is the heterozygous 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. 



In Drosophila the female is apparently homozygous as 

 regards some cell structure, X, which in the male is never 

 represented more than FEMALE 

 once. Accordingly, 



(X-RIX-R) 



the formula of the 

 female is in such cases 

 XX; that of the male, 

 X . Now the sex- 

 limited characters in 

 Drosophila seem to be 

 bound up with the X 

 structure, not repelled 

 by it, as is barring in 

 fowls. Accordingly, a 

 sex -limited character 

 maybe represented 

 twice in the female 

 Drosophila, but only 

 once in the male; or in other words, the female may be 

 homozygous as regards a sex-limited character, but the 

 male can only be heterozygous (Fig. 38). 



Drosophila normally has red eyes, but the redness of 

 the eye is a distinct unit-character, sex limited in heredity. 

 Further, males are regularly heterozygous in this character, 

 while females are homozygous. For Morgan has obtained 

 a race in which the eyes are white, owing to the loss of the 

 red character; and reciprocal crosses of this race with 



FIG. 38. Diagram of sex-limited inheritance 

 when the female is a homozygote, as in the red- 

 eyed Drosophila. X, sex determiner; R, red 

 eyes. 



