Sex Determination 179 



is reduced one-half. It is obvious that in the female 

 each egg receives one sex chromosome, and therefore 

 all eggs are alike in this feature. In the male, however, 

 with one sex chromosome, at the reduction division 

 the solitary sex chromosome goes to one pole, leaving 

 the other pole without such a chromosome. As a 

 result there are two kinds of sperms, one half containing 

 a sex chromosome, the other half containing none. At 

 fertilization, if an egg mates with a sperm having a sex 

 chromosome the zygote contains two, and this results 

 obviously in a female, for females are characterized by 

 two sex chromosomes. With a sperm of the other type, 

 the zygote receives only one sex chromosome and must 

 produce a male individual. As a result males and 

 females are produced in equal numbers, and sex is 

 determined by the type of sperm that enters into the 

 sex fusion. 



In such cases females are homozygous for sex and 

 males are heterozygous, for in the male the sex chromo- 

 some is paired with its absence. This is the commonest 

 situation, although it would be theoretically possible to 

 have females heterozygous for sex, having the single 

 chromosome, while the male had a pair. In this case 

 sex would be determined by the type of egg that entered 

 into the sex fusion. In addition, there are cases where 

 one sex has one sex chromosome and the other sex has 

 none. Finally, there are cases in which one sex has a 

 pair of large chromosomes, while the other sex has one 

 large and one small sex chromosome. In all of these 

 cases, however, the fundamental mechanism is the same. 

 Either male or female (commonly the former) is hetero- 

 zygous for sex, producing two kinds of gametes, as 



