CHAPTER XIV 



SEX AND GENES 



OUR present understanding of the mechanism of 

 I sex-determination has come from two sources. 

 Students of the cell have discovered the role 

 played by certain chromosomes and students of genetics 

 have gone further and have discovered important facts 

 as to the role of the genes. 



Two principal types of mechanism for sex-determina- 

 tion are known. They both involve the same principle, 

 although they may seem, at first, to be the converse of 

 each other. 



The first type may be called the insect type, because in 

 insects we have the best cytological and genetic evidence 

 for this kind of sex-determining mechanism. The second 

 type may be called the avian type, because in birds we 

 now have both cytological and genetic evidence for this 

 alternative mechanism. It is also present in moths. 



The Insect Type (XX-XY). 



In the insect type the female has two sex-chromosomes 

 that are called X-chromosomes (Fig. 109). When the eggs 

 of the female ripen (that is, after each has given off its 

 two polar bodies), the number of the chromosomes is re- 

 duced to one-half. Each ripe egg, then, contains one X 

 and, in addition, one set of ordinary chromosomes. The 

 male has one X-chromosome only (Fig. 112). In some 

 species this X has no mate ; but in other species it has a 

 mate that is called the Y-chromosome (Fig. 113). At one 

 of the maturation divisions the X and the Y pass to oppo- 



