102 EVOLUTION AND GENETICS 



45 and 46, illustrating crosses between a white-eyed 

 and a red-eved individual. 



The first of these represents a cross between a 

 white-eyed male and a red-eyed female {fig. 45, top 

 row) . The X-chromosome in the male is represented 

 by a bar, (w), the I^-chromosome is bent. In the fe- 

 male the A"-chromosomes are W and W. Each egg 

 of such a female will retain one X (with W) 

 after the polar bodies have been thrown off. In 

 the male there are two classes of sperm — the female- 

 producing, carrying JT (with w) ; and the male- 

 producing, carrying the I^-chromosome. Any egg 

 fertilized by an X-bearing sperm w^ill produce a 

 female with red eves, because the A"-chromosome 

 (W) from the mother carries the dominant fac- 

 tor for red. Any egg fertilized by a I^-bearing sperm 

 will produce a male with red eyes because he gets his 

 X-chromosome (W) from his mother. 



When these two Fi flies (second row) are inbred 

 the following combinations are expected. Each egg 

 will contain a red-eye producing X, ( W ) , or a 

 white-eye producing X, (w) , after the polar bodies 

 have been extruded. The male will produce two 

 kinds of sperms, of which the female-producing 

 will contain a red-eye producing X. Since any 

 egg may by chance be fertilized by any sperm, there 

 will be the four classes of individuals shown in the 

 bottom row of the diagram. All the females will 

 have red eyes, because irrespective of the two kinds 



