HEEEDITY 



N — 1 or N — 2 (see Fig. 50). In consequence 

 zygotes of two sorts result, — those which are 

 2 N, female, and those which are 2 N — 1 or 

 2 N — 2, male. Thus in the squash-bug, Anasa- 

 tristis, according to Wilson, the mature egg 

 contains 11 chromosomes, the spermatozoa 

 either 10 or 11 chromosomes, the two sorts 

 being equally numerous. 



Egg 11 + sperm 11 produces a zygote 22 (2N), a female; 

 Egg 11+ " 10 " " " 21(2N-l),amale. 



N in this species = 11 ; 2 N = 22, the female ; 

 2 N — 1 = 21, the male. Males and females are 

 therefore approximately equal in number, as in 

 most animals where the two sexes are not sub- 

 ject to unequal mortality. In the Mendelian 

 sense the female is in such cases a homozygote, 

 the male a heterozygote. The sex of an indi- 

 vidual in such cases depends upon which sort 

 of a sperm chances to enter the egg. 



But the experimental evidence indicates 

 that both as regards sex and as regards 

 heritable characters correlated with sex, these 

 relations may in some cases be reversed, the 

 female being heterozygous, the male homozy- 



168 



