68 



HEREDITY AND SEX 



These inbred gave gray males and females and yellow 

 males and females, in equal numbers. 



These cases serve to illustrate the regularity of this 

 type of inheritance and its relation to sex. In the fruit 

 fly we have found as many as twenty-five sex-linked 



X 



XM 



Ir 



w 



X( 



Fig. 39. — Reciprocal cross of Fig. 38. Parents, long-winged Z and 

 short-winged 9 . Fi long-winged 9 . short-winged Z • F2 long-winged 

 9 and Z , short-winged 9 and $ . Sex chromosomes as in last. 



factors. There are other kinds of inheritance found in 

 these flies, and at another time I shall speak of some 

 of these ; but the group of sex-linked factors is of special 

 importance because through them we get an insight 

 into the heredity of sex. 



In the next chapter, when we take up in detail Men- 

 delian heredity, I shall try to go further into the ex- 



