THEORY OF EVOLUTION 121 



there will be two classes of sperm the female- 

 producing, carrying the (open) X, and the 

 male-producing, carrying the Y chromosome. 

 Any egg fertilized by an X bearing sperm will 

 produce a female that will have red eyes be- 

 cause the X (black) chromosome it gets from 

 the mother carries the dominant factor for red. 

 Any egg fertilized by a Y-bearing sperm will 

 produce a male that will also have red eyes be- 

 cause he gets his (black) X chromosome from 

 his mother. 



When, then, these two FI flies (second row) 

 are inbred the following combinations are ex- 

 pected. Each egg will contain a black X (red 

 eye producing) or a white X (white eye pro- 

 ducing) after the polar bodies have been ex- 

 truded. The male will produce two kinds of 

 sperms, of which the female producing will 

 contain a black X (red eye producing) . Since 

 any egg may by chance be fertilized by any 

 sperm there will result the four classes of indi- 

 viduals shown on the bottom row of the dia- 

 gram. All the females will have red eyes, 

 because irrespective of the two kinds of eggs 

 involved all the female-producing sperm carry 



