THE PRINCIPLES OF HEREDITY 519 



case where a mature man was transformed into a rather convincing 

 replica of a woman by testes removal and injections of the female sex 

 hormones. 



Hormones cannot offer a complete explanation for sex determina- 

 tion, for there still must be some factor to start sex in one direction or 

 the other. We must go back to the chromosomes to find this. When 

 we examine the chromosomes within the cells of the body of a woman 

 we find 24 perfect pairs, but in the cells of a man we can find only 23 

 matched pairs and one pair which is unequal in size. The large one 

 of this unequal pair is called the X-chromosome and the small one the 

 Y-chromosome. These are known as the sex chromosomes. One of 

 the 24 pairs in the female consists of two X-chromosomes. Experiments 

 with a number of different kinds of animals show that this chromosome 

 difference is the basis for determining sex. Two X-chromosomes estab- 

 lish a balance with the other chromosomes which favors the female- 

 determining genes, while a single X-chromosome establishes a balance 

 which favors the functioning of the male-determining genes. The Y- 

 chromosome seems to play no part in sex determination. 



In spermatogenesis, the 48 chromosomes are separated so that one 

 half of the sperms have an X-chromosome and the other half have a 

 Y-chromosome as one of their 24. The eggs, of course, all bear an 

 X-chromosome as one of their 24. When an X-bearing sperm fertilizes 

 the egg the zygote will have two X-chromosomes and a girl will result. 

 A Y-bearing sperm on the other hand results in the XY combination 

 in the zygote and this produces a boy. Since these two types of sperms 

 are produced in approximately equal numbers, the balance of the sexes 

 is maintained. 



Only the vertebrates appear to have sex hormones which play their 

 part in sex determination as we have described it. Among most of the 

 invertebrates with separate sexes, however, the chromosome mechanism 

 operates to do the job alone. In the fruit fly, Drosophila, for instance, 

 there is the XY method of determination as in man. There are a few 

 forms of life, however, in which there is a reversal of the chromosome 

 arrangement. In birds, moths, butterflies, and some fish the unlike pair 

 of sex chromosomes is found in the female and the like pair in the male. 

 Thus, the sperms are all alike and the eggs are of two different types 

 with regard to sex chromosomes. In order to avoid confusion, these 

 are sometimes called WZ in the female and ZZ in the male. The honey- 

 bee and a few similar insects have a distinctive method of sex determina- 

 tion — fertilized eggs develop into females and unfertilized eggs develop 

 into males (see Chapter 16). 



