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 6 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 (0X) (6X) 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 Y-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 F; 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 Ff’; women of the genetic 
constitution (BX)(bX), however, when mated to normal men produce 
daughters of the formule (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 
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