MENDELIAN SEGREGATION 15 



separation of the X chromosomes in the female there is 

 one X chromosome left in each egg. In the male there 

 is one X chromosome and another chromosome, its 

 mate, called the Y chromosome. Hence in the male 

 there are two classes of spermatozoa : one containing 

 X, the other Y. If a Y-bearing spermatozoon should 



FEMALE MALE 



89 86^ 



EGG 



rsr^ 



SPERM 



Gametes 



98 



FEMALE MALE 



Fig. 8. — Diagram to show the history of the sex chromosomes from 



one generation to the next. 



fertilize an egg the result will be an XY individual, 

 or male. It is evident that the Y chromosome is 

 found only in the males, while an X chromosome 

 passes not onh^ from female to female, but also from 

 female to male and from male to female. 



As will be shown now, certain factors follow the 

 distribution of the X chromosomes and are there- 



