188 EVOLUTION AND GENETICS 



woman who had a color-bhnd father, or had a 

 mother heterozygous for color-blmdness {fig. 70) . 



The pedigrees of color-bhnd famihes — and they 

 are many — leave little doubt as to the mode of in- 

 heritance of this character {fig. 71). 



Accepting this evidence as on the whole satisfac- 



XX? XY(/ 



XI? 



XYc/ 



Fig. 69. — Diagram to show the inheritance of color-blindness 

 in man. The eye that can distinguish red from green is here half 

 black, half barred, while the color-blind eye is stippled. A color- 

 blind man mates with a normal woman. The sons and daughters 

 are normal. Two individuals of such i)arentage give three nor- 

 mals to one color-blind individual in l\. The color-blind indi- 

 vidual is always a male. 



tory, there is still something more to be said. As is 

 well known, there are many grades of color-blindness. 

 We do not know whether these grades are due to 

 individual, non-genetic, variations — assuming it to 



