264 Experimental Transformation of Species 



The " Knight's move " pattern of hereditary transmis- 

 sions, now explained as resulting from a sex-linked deter- 

 miner, has been observed for other characters. Notable 

 among human beings is a certain type of " bleeding," due to 



Eye characters 



Chromosomes 





Parents 



\sJ\ ^ Gametes 

 t t ? 



Fi 



ILX? ^ Gametes 



m M m xo F2 



9 9 cf cf 



Fig. 75. Inheritance of Color-blindness 



The sex chromosomes in the male are represented by XO and those 

 in the female by XX. The male produces two kinds of sperms, those 

 with the X chromosome and those without; but all eggs contain one 

 X chromosome. There are therefore two possible sperm-egg combina- 

 tions, which would theoretically occur in equal numbers, resulting in 

 approximately equal numbers of males and females in the Fj genera- 

 tion. Assuming that color-blindness is determined by something in the 

 X chromosome of the color-blind male, half of the sperms (those with 

 the X chromosome) would carry the determiner for this character. 

 In the Fi generation all of the females will carry this chromosome, but 

 none of the males. Females, however, do not show color-blindness if 

 one of their X chromosomes is free from the determining element. In 

 this generation the males again produce two kinds of sperm cells, but 

 neither can carry the determiner for color-blindness. The females, 

 however, produce two kinds of eggs, half of the X chromosome carry- 

 ing the determiner for color-blindness. There are therefore possible 

 two kinds of males in the F., generation, those with and those without 

 the affected chromosome. But since the male exhibits color-blindness 

 when carrying a single affected X chromosome, half the males are 

 color-blind; and there are no color-blind females. 



a defective clotting of the blood. Like color-blindness this 

 hsemophilia occurs in males and is transmitted through 

 daughters (who do not exhibit the trait) to grandsons. 

 Like color-blindness also, haemophilia occurs in females only 



