THEORY OF EVOLUTION 127 



divisions the X chromosome passes to one pole 

 undivided (fig. 61, C). In consequence there 

 are two classes of sperms in man ; female pro- 

 ducing containing 24 chromosomes, and male 

 producing containing 23 chromosomes. If the 

 factor for color blindness is carried by the X 

 chromosome its inheritance in man works out 

 on the same chromosome scheme and in the 

 same way as does white eye color (or any other 

 sex linked character) in the fly, for the O 

 sperm in man is equivalent to the Y sperm in 

 the fly. 



In these cases we have been dealing with a 

 single pair of characters. Let us now take a 

 case where two pairs of sex linked characters 

 enter the cross at the same time, and preferably 

 a case where the two recessives enter the cross 

 from the same parent. 



If a female with white eyes and yellow wings 

 is crossed to a wild male with red eyes and gray 

 wings (fig. 62), the sons are yellow and have 

 white eyes and the daughters are gray and 

 have red eyes. If two FI flies are mated they 

 will produce the following classes. 



