INHERITANCE OF SEX AND RELATED PHENOMENA 197 



cases on favorable cytological evidence, but in only a few cases has 

 satisfactory evidence been secured from both sources. 



In a previous chapter we have shown in detail how a sex-linked char- 

 acter in Drosophila is inherited. By referring to the list of factors in 

 Drosophila it may be seen that in this insect about fifty factors are known 

 to belong to the first chromosome, and, therefore, to display the sex- 

 linked type of inheritance. Although cases of sex-linked inheritance 

 are known in other animals, in none do we have as complete a body of 

 knowledge as in Drosophila. Nevertheless, there is a sufficiency of 

 other cases to lend strong support to the evidence derived from the Dro- 

 sophila investigations. In man particularly several sex-linked factors 

 are known, and the evidence in support of this analysis is fairly 

 satisfactory. 



A typical case in man is that of color-blindness, which is much more 

 common in males than in females. The factor for color-blindness may 

 be called b and its normal allelomorph B. A normal-visioned woman is 

 then of the genetic constitution (BX)(BX), and a normal man is (BX)Y. 

 The corresponding abnormal forms are for women (bX) (bX) and for men 

 (bX) Y. Since the factor for color-blindness is recessive, a woman of the 

 genetic constitution (BX) (bX) will have normal color vision. In this we 

 see the reason for the greater number of men that are color-blind. A man 

 with a simplex dose of the factor is color-blind, because the F-chromosome 

 as in Drosophila carries no demonstrable factors. In the simplex woman, 

 (BX)(bX), on the other hand, the dominant allelomorph determines the 

 type of color vision, so that a normal woman is produced. Simplex 

 women are just as common as simplex men, the greater number of men 

 displaying the color blind character is simply due to the different chromo- 

 some constitutions of the two sexes. 



The relations which exist in the inheritance of color-blindness are 

 exactly the same as those which exist in the inheritance of white eye color 

 in Drosophila. A normal woman (BX)(BX) mated to a color-blind 

 man (bX) Y produces in FI normal daughters of the genetic constitution 

 (BX) (bX) and normal sons of the genetic constitution (BX) Y. These FI 

 normal sons are of exactly the same genetic constitution as all other nor- 

 mal men and, therefore, although they had a color-blind father, they 

 can never transmit the defect. The normal FI women of the genetic 

 constitution (BX)(bX), however, when mated to normal men produce 

 daughters of the formula (BX)(BX) and (BX)(bX) } all of which are, 

 therefore, normal, and sons in equal numbers of the constitution (BX) Y, 

 normal, and (bX) Y, color-blind. A simplex woman, therefore, although 

 she does not herself exhibit the color-blind character, when mated to a 

 normal man, transmits that character to none of her daughters, but to 

 half of her sons. A color-blind woman can be produced by the rare 



