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 
BBY 
88 Be 
FEMALE 
Fic. 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 only 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- 
