170 PHYSICAL BASIS OF HEREDITY 



are bred together, the results are as follows : Half the 

 daughters have white eyes (ww), half red eyes (wW); 

 half the sons have white eyes (w), half red eyes (W). It 

 will be seen that the red-eyed Fg daughters are all hetero- 

 zygous, and should give 50 per cent, white and 50 per cent, 

 red offspring if mated to white-eyed males. This occurs. 



Similar illustrations might be given, for any of the 

 50 sex-linked characters of Drosophila. Of these the sex- 

 linked lethals form the most interesting cases and will be 

 spoken of in another connection. 



Despite the fact that the results in one of the two fore- 

 going crosses gave a 3 : 1 ratio, and in its reciprocal a 1 : 1 

 ratio, the results in both cases conform to Mendel's first 

 law of segregation. The peculiarity of the 1 : 1 ratio is 

 due to the fact that the P^ red-eyed male is in a sense 

 heterozygous for the wild-type eye color (since he has 

 but one Z-chromosome that, carries the factor for red 

 eyes). Since in the second cross jthe Fj male gets no red- 

 producing X from either parent^ he is pure for white 

 eyes in the sense that he has an X bearing the factor for 

 white eyes and a Y that bears no factor making red. 

 Hence this Fi cross is exactly like a back-cross of a hetero- 

 zygous female to a recessive male, and gives the same 

 numerical result, viz., 1 : 1. 



Cases of sex-linked inheritance of this kind are also 

 known in man. Color blindness in man appears to follow 

 exactly the same procedure as sex-linked inheritance in the 

 vinegar fly — at least certain kinds of color blindness have 

 been shown to do so. Haemophilia also is sex-linked, and 

 there are four or five other defects in man that appear 

 to come under this head. According to several accounts 

 there is an unpaired sex-chromosome (or two of them) 

 in man, which is also called for by the genetic evidence 

 relating to sex-linkage in man, but since .the female number 

 of chromosomes in man is stated by Guyer to be 24, and 

 by von Winiwarter to be 48, it is unsafe as yet to appeal 



