GENETICS 



235 



female produces only one kind of egg {w); but the male, because of his 

 Y chromosome which lacks any gene of this pair, produces two kinds of 

 spermatozoa, one having the X chromosome (with w), the other the Y 

 chromosome. The two combinations of eggs and spermatozoa produce 

 the two sexes, respectively, of the Fi generation. The males of this 

 generation are white-eyed because there is no red gene (W) to dominate 

 over their white gene. In the F2 generation, as the figure shows, there 

 are four combinations, two of which are red, two white. The marks of 

 sex-linkage in tliis cross are (1) that the Fi generation is of two kinds. 



Fig. 203. — Sex linkage of eye color in Drosophila. Left, white-eyed female X red-eyed 

 male. Right, the reciprocal cross. (Modified from Morgan, Sturtevant, Muller, and Bridges, 

 "Mechanism of Mendelian Heredity," Henry Holt and Company, Inc.) 



instead of only the dominant type, and (2) that the F2 ratio of dominant 

 to recessive is 1:1 instead of 3:1. 



If the cross is made with the red eyes in the female and white eyes in 

 the male, the results shown are as indicated at the right in Fig. 203. Tht 

 Yi males get their eye color gene from their mother as before but now are 

 red-eyed, as are also the heterozygous Fi females. In the F2 generation 

 there are again four combinations. Three of these are red-eyed; hence 

 the F2 ratio is 3 red: 1 white. However, the white-eyed F2 f^ies are all 

 males. This last feature is the only sign, when the cross is made this 

 way, that the character being studied is sex-linked. 



Any animal or plant whose sex is determined by chromosomes, and in 

 which, as a consequence of this chromosome relation, the male produces 

 two kinds of spermatozoa, may be expected to show sex-linkage of the 

 kind just i"'istrated. Man is one of these animals. A modified form of 



