Inheriting Sex 257 



characteristics are always associated with the presence of sex- 

 determining chromosomes. 



Inheriting Sex 



To say that the individual inherits his sex is today com- 

 monplace among biologists. To those who are not acquainted 

 with the underlying facts it still sounds absurd. From which 

 of the two parents does the individual inherit his or her sex? 

 It will no doubt add to the incredibility of the statement to 

 say that in some kinds of animals sex is inherited from the 



Female Male 



Fig. 69. The Chromosomes of the Fruit Fly 



There are two pairs of large chromosomes, somiewhat bent; one 

 pair of very small ones; and one pair of intermediate, rod-shaped 

 chromosomes, making four pairs in all. This last pair differs in the 

 two sexes, the female having two that appear to be alike, whereas 

 in the male one is a little longer and bent at the end. After Bridges. 



father, whereas in other species it is inherited from the 

 mother. But that is indeed the case. 



In the female Drosophila there are four pairs of chromo- 

 somes; and the members of each pair are indistinguishable 

 (Fig. 69). In the cells of the male, the chromosomes are 

 just the same except that one of the medium-sized pair is 

 longer than the other and bent near the end. This differ- 

 ence between the sexes is constant. There is something about 

 these chromosomes that determines the sex of the developing 

 individual. The straight chromosome has been called the X 

 chromosome, and the bent one the Y chromosome. In the 

 presence of XX, the fly is a female; in the presence of XY, 

 it is a male. 



