Sex-linked Lethal Factors in Man. 113 



Especially interesting are the relations of the colors in the two 

 sexes. It will be noted that in the tortoise-shell females neither 

 the yellow factor nor the black factor are completely dominant. 

 They both have a share in the color of the individual and may 

 therefore be considered as balancing each other's activity to some 

 extent. In the male however, the black factor is not balanced by 

 any factor for yellow and thus produces a black coat. 



In man it has been shown that several characters, among 

 which are hemophilia and color blindness, depend primarily upon 

 mendelizing units carried in the X or sex chromosome. Their 

 inheritance would, if the hemophilic or color blind condition is 

 recessive, be as follows : 



H = normal, 

 h = hemophilia. 

 HXHX normal 9 mated with hX — hemophilic o 71 : 

 Offspring; HXhX — normal female (carrying recessive hmo- 

 philia). 

 HX— normal male. 

 HXhX heterozygous normal 9 mated with HX — normal cf . 

 HXHX = normal female. 



HXhX= normal female (carrying hemophilia). 

 HX— = normal male. 

 hX — = hemophilic male. 

 Lethal factors if sex-linked, follow the same type of inheritance. 

 Thus if L = normal, 1 = lethal. 



LX1X = normal female heterozygous, crossed with LX — 

 normal male. 



LXLX = normal female. 

 LX1X = normal female (heterozygous). 

 LX— = normal male. 

 IX— = lethal male (dies). 

 Since any sex-linked lethal factor in man would, by hypothesis, 

 be borne in the same chromosome as the factor for hemophilia or 

 for color blindness or their normal allelomorphs, it would be linked 

 to them in inheritance. If it were closely linked with the normal 

 allelomorph of hemophilia it would cause the death of the great 

 majority of the normal males which in its absence should occur 

 in equal proportions to hemophilic males. (See diagram above.) 



