THE MECHANISM OF REPRODUCTION 47 



those regarding characters commonly known as sex- 

 linked, whose distribution can be accurately predicted 

 if we assume they are definitely coupled with the 

 sex determiner. 



Such a character is hereditary color-blindness in man, 

 a condition in which the affected individual cannot dis- 

 tinguish between red and green. It is far commoner in 

 man than in women, and its inheritance is so peculiar that 

 it often seems to skip a generation. 



A color-blind man married to a normal woman will 

 have only normal children of either sex. The sons will 

 never have color-blind progeny by women with normal 

 vision, but the daughters, though married to normal men, 

 will transmit color-blindness to one-half of their sons. 

 If, moreover, a daughter mates with a color-blind man, 

 as might frequently happen in marriage between cousins, 

 on the average one-half of her daughters as well as one- 

 half of her sons will be abnormal. 



This interesting and apparently complicated inheri- 

 tance is really very simple if we merely assume that the 

 sex chromosomes of the color-blind individuals also carry 

 the determiner for color-blindness. Fig. 16 shows what 

 would be expected. Representing the normal vision by 

 boldface type and color-blindness by outline we see first 

 the result of mating a normal woman with a color-blind 

 man. Since all of her sex-cells, when matured, contain 

 one normal x-element, and since the sex-cells of the 

 male are of two kinds, half containing an abnormal or 

 color-blind determining x-element and half containing no 

 x-element whatever, it is obvious that the sons must re- 

 ceive their x-element only from their mother and the 

 daughters must receive one of their x-elements from their 



