THEORY OF EVOLUTION 119 



fEllALE MALE 



89 M 



/ \ [•GiMETES 



SPOiri 



88 8ff 



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 

 any sperm will give either XX (female) or XY (male). 



linked inheritance are shown in the next dia- 

 gram of Drosophila illustrating 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. 

 58, 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 oflp. In the male 



