THEORY OF EVOLUTION 119 



FEMALE MiLE 



88 M 



8 



Fig. 57. Scheme of sex determination in Drosophila type. 

 Each mature egg contains one X, each mature sperm contains 

 one X, or a Y chromosome. Chance union of any egg with 

 an\' sperm will give either XX (female) or XY (male). 



linked inheritance are shown in tlie next dia- 

 gram of Drosophihi ilhistrating a cross between 

 a white eyed male and a red eyed female. 



The first of these represents a cross between 

 a white eyed male and a red eyed female (fig. 

 o8. top row). The X chromosome in the 

 male is represented by an open bar, the Y 

 chromosome is bent. In the female the two X 

 chromosomes are black. Each egg of such a 

 female will contain one "black" X after the 

 polar bodies have been thrown off. In the male 



