11 SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 



ance. It conforms to the general scheme of inheritance in other 

 animals; in Drosophila, for example, we have about sixty mutant 

 characters which show this form of inheritance. 



A color-blind man married to a normal woman has only nor- 

 mal daughters and sons; all of the daughters, however, transmit 

 color-blindness to half of their sons, Fig. 6. 



Color-blind women are rare, because they can never arise un- 

 less a color-blind man marries a woman who is color-blind, or 

 else marries a normal woman who had a color-blind father, or 

 had a mother heterozygous for color-blindness, Fig. 7. 



XYd* 



Fig. 7. Reciprocal of the cross shown in Fig. 6. Here a normal-eyed 

 male marries a color-blind female, giving all color-blind sons and normal 

 daughters. When two individuals like these marry, the expectation is for 

 half of the daughters and half of the sons to be color blind, and half of the 

 daily liters and half of the sons to be normal eyed. 



The pedigrees of color-blind families — and they are many — 



leave little doubt as to the mode of inheritance of this character. 



Accepting this evidence as on the whole satisfactory, there is 



