292 FOUNDATIONS OF BIOLOGY 



the other without the X chromosome half of the sperm 

 contain an X chromosome. 



Furthermore, in species in which the male has the X 

 chromosome, the female has two of them. The female 

 therefore has one more chromosome than the male. Thus 

 during oogenesis the X chromosomes pair in synapsis just 

 as the other homologous chromosomes, and then one is dis- 

 tributed to each of the daughter cells, so that all of the eggs 

 contain an X chromosome. For instance, in Man the somatic 

 number of chromosomes apparently is forty-seven in males, 

 or forty-six plus the X chromosome; while the female somatic 

 number is forty-eight, or forty-six plus two X chromosomes. 

 Half of the sperm contain 23 and half 24 chromosomes; all 

 the eggs contain 24 chromosomes. 



Since there are equal numbers of sperm with and without 

 the X chromosome, on the average as many eggs will be 

 fertilized by one class of sperm as the other, with the result 

 that half of the zygotes will contain one X and half two X 

 chromosomes. Obviously the former will develop into 

 males and the latter into females, since the somatic cells 

 of males have the X chromosome and therefore the 'sex 

 gene' in simplex condition, and similar cells of females have 

 the duplex condition. So it is possible it has been accom- 

 plished in several species to ascertain the sex of an 

 embryo by counting the chromosomes in its cells. (Fig. 148.) 



Thus there is good cytological evidence that sex inheri- 

 tance follows the Mendelian formula. The male carries one 

 sex gene (on the single X chromosome) and the female two 

 sex genes (one on each of the X chromosomes) . At matura- 

 tion these segregate so that the male is heterozygous and the 

 female is homozygous in regard to sex, and therefore all 

 possible combinations of gametes result in the 1 : 1 ratio of 

 males to females. In passing, we may emphasize that this 



