100 Genes in the X and Y Chromosomes 



recessive genes. If a color-blind man marries a normal homozy- 

 gous woman, all his offspring will be normal, although his daugh- 

 ters will be heterozygous and will therefore be ''carriers." It is 

 only when a color-blind man marries a "carrier" that an indi- 

 vidual with two recessives can be formed. Such an individual 

 would be a color-blind woman. The number of carriers and the 

 number of color-blind men are small when compared with 

 normal people. Therefore, if mating is at random, as it usually 

 is, the chance that a carrier would marry a color-blind man is 

 very small. Since even then such a marriage does not neces- 

 sarily produce a daughter, and since only half the daughters 

 would be expected to be color blind, the chance that a color- 

 blind woman would be produced is extremely small. If a small 

 community became settled by several families one of which 

 carried the color-blind gene, and if this community was so 

 isolated by geographical, racial, religious, or other factors that 

 there tended to be a great deal of intermarriage within the com- 

 munity, the chance that a color-blind woman would appear 

 would be much greater than in the usual situation of purely 

 random mating. However, even in such a community, more 

 men than women would be color blind. 



Hemophilia. Hemophilia is another sex-linked human char- 

 acter which is caused by a recessive gene. Individuals possessing 

 this character show a deficiency in the power of their blood to 

 clot, with the result that they may bleed to death from a cut 

 which, in a normal person, would be regarded as trivial. Conse- 

 quently, people with this character are ''bleeders" and frequently 

 fail to live to reach their twenties. Just as they are less likely 

 to be color blind, and for the same reason, women are much less 

 likely than men to have hemophilia. In fact, it has generally 

 been questioned whether such women do or even could exist. 

 Snyder examined 250 published pedigrees of hemophilia and 

 found only one in which a known hemophilic male was married 

 to a known carrier. In this family and one from his own records 

 there were three normal daughters and two hemophilic sons. 

 Since half the daughters from that type of mating would be 

 expected to be normal, these two families are inconclusive. The 

 question of whether women can have hemophilia is unsolved. 

 The chances are extremely small and no known hemophilic 

 woman has yet been discovered and called to the attention of 



