104 EVOLUTION AND GENETICS 



of eggs of the female, all the female-producing sperm 

 carry a (w) X, Half of the males have red eyes, 

 because half of the eggs had each a red-producing 

 X-chromosome. The other half of the males have 

 white eyes, because half of the eggs had each a white- 

 producing A^-chromosome. Evidence from other 

 sources shows that the I^-chromosome of the male is 

 indifferent, so far as these Mendelian factors are 

 concerned. 



The reciprocal experiment is illustrated in jigure 

 46. A white-eyed female is mated to a red-eyed 

 male (top row) . Each of the mature eggs of such a 

 female contains one white-producingX-chromosome, 

 represented by the open bar in the diagram. The red- 

 eyed male contains female-producing X-bearing 

 sperm, that carry the factor for red-eye color, and 

 male-producing I^-chromosomes. Any egg fertilized 

 by an A"-bearing sperm will become a red-eyed 

 female because the X-chromosome that comes from 

 the father carries the dominant factor for red eye 

 color. Any egg fertilized by a I^-bearing sperm will 

 become a male with white eyes because the only X- 

 chromosome that the male contains comes from his 

 mother and is white-producing. 



When these two F^ flies are inbred (middle row) 

 the following combinations are expected. Half the 

 eggs will contain each a white-producing X-chromo- 

 some and half a red-producing. The female-producing 

 sperms will each contain a white X- and the male- 



