INHERITANCE OF SEX 



225 



some of the male (X), unites at synapsis with a smaller element 

 (Y) the end stages of maturation giving again two types of 

 spermatozoa, one type containing the sex chromosome (X) 

 the other its smaller mate (Y) (Fig. 100). The female on the 

 other hand contains the full number of chromosomes including 

 two X's. After maturation there is one X in each egg. Fer- 



xx 



X0XX 



FIG. 101. Sex-linked inheritance of white and red eyes in Drosophila. 

 Parents, white-eyed d" and red-eyed 9 ; Fi, red-eyed & and 9 ; F 2 , red-eyed 9 , 

 red-eyed d" and white-eyed d". To right of flies the history of the sex chromo- 

 somes XX is shown. The black X carries the factor for red eyes, the open X 

 the factor for white eyes, O stands for no X. (From Morgan.) 



tilization results in embryos with either one X or two X's. In 

 the former case the individual is a male, in the latter case a 

 female. 



In still other types of bugs the corresponding X and Y 

 chromosomes are of equal size and cannot be distinguished 

 morphologically but males and females are produced in equal 

 numbers and the conclusion is justified that one of the chromo- 

 somes is the sex determining or X chromosome (see Ascaris, 

 Fig. 91). 



