THEORY OF EVOLUTION 123 



a black X. Half of the males have red eyes 

 because half of the eggs have had each a red- 

 producing X chromosome. The other half of 

 the males have white eyes, because the other 

 half of the eggs had each a white-producing X 

 chromosome. Otlier evidence has sliown that 

 the Y chromosome of the male is indifferent, so 

 far as these Mendelian factors are concerned. 

 The reciprocal experiment is illustrated in 

 figure 59. A white eyed female is mated to a 

 red eyed male (top row) . All the mature eggs 

 of such a female contain one white-producing 

 X chromosome represented by the open bar 

 in the diagram. The red eyed male contains fe- 

 male-producing X-bearing sperm that carry 

 the factor for red eye color, and male-produc- 

 ing Y chromosomes. Any egg fertilized by an 

 X-bearing sjDerm 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 Y-bearing 

 sperm will become a male with white eyes be- 

 cause the only X chromosome that the male 

 contains comes from his mother and is white 

 producing. 



