292 



THE CONTINUITY OF THE RACE 



As soon as the correlation between sex and chromosomes was dis- 

 covered, it was seen that such a mode of sex determination gave further 

 evidence of the residence of the genes within the chromosomes. Long 

 before Mendel's experiments were rediscovered, a peculiar type of in- 

 heritance had been noted in man, in which certain recessive traits were 

 most frequently transmitted to the sons but not to the daughters, by a 

 normal-appearing mother. Color blindness, the inability to distinguish 

 red from green, is such a character; one type of night blindness, the in- 

 ability to see in dim light, is another; and inherited hemophilia, a condi- 

 tion in which the blood lacks the ability to clot, is still another. The 

 peculiar mode of inheritance of these characters is clearly accounted for 

 if we suppose that the genes for color blindness, night blindness, and 

 hemophilia and their dominant alleles are located in the X chromosome. 

 This is shown by the accompanying diagram (Fig. 19.1), illustrating the 

 inheritance of color blindness, in which the X chromosomes are repre- 

 sented as oval, the Y chromosomes as oblong, N is the gene for normal, 

 and n the gene for color blindness. 



Similar cases of sex-linked inheritance are well known in Drosophila. 

 In poultry and in Lepidoptera the characters found to be sex-linked show 

 an inheritance in exact keeping with the fact that in these groups the male 

 has two X chromosomes and the female but one. 



LIMITATIONS TO MENDEL'S SECOND LAW 



Until 1906, all the rapidly accumulating crosses that involved two 

 or more pairs of allelomorphs showed the independent assortment postu- 

 lated by Mendel's second law. Some of the adherents of the Sutton- 



