THE INHERITANCE OF EYE-COLOR 607 



traits do show a sharp discontinuity and are consequently 

 more easily studied. Altogether about fifty traits have 

 been sufficiently studied to know that they are inherited 

 and in some cases to know definitely how. Among nor- 

 mal characters may be mentioned: color of hair, eyes, 

 and skin; form of hair whether straight, curly, or kinky; 

 stature, proportion, and weight of the body. Hands and 

 feet with two-jointed fingers or toes or with six digits 

 in place of five, web feet, color blindness (inability to 

 distinguish red from green most common), night blind- 

 ness (inability to see in dim light), hoemophilia (a cer- 

 tain type of persistent bleeding), and cretinism (a disease 

 due to deficiency of the thyroid gland hormone) are 

 among the better known. 



The Inheritance of Eye-Color. — The color of the 

 eye (Fig. 144) depends on pigment granules deposited 

 in the iris. All eyes have some pigment on the back 

 side of the iris, which pigment when present alone 

 gives pure blue eyes. Non-blue eyes have in addi- 

 tion to this pigment more or less brown pigment in 

 the cells forming the front side of the iris. The vary- 

 ing shades of eye-color from gray to black depend on 

 the amount and distribution of this second pigment on 

 the front. If a blue-eyed person marries a pure dark- 

 eyed one all the children will have dark eyes. If these 

 dark-eyed hybrid children marry blue-eyed mates one- 

 half their children (i.e., the F2) will be dark-eyed and 

 one-half blue-eyed. If the dark-eyed hybrids marry 

 other dark-eyed hybrids (i.e., the offspring of a blue- 

 eyed by a dark-eyed parent) their children (i.e., the Fa) 

 will show three dark-eyed to one blue-eyed, approxi- 

 mately. This is clear and sufficient proof to show that 

 this eye-color difference depends on only one pair of 

 factors or genes. It behaves exactly like coat-color in 

 guinea pigs or flower color in plants. 



Brachydactyly is a condition of the hands in which 

 they appear proportionally very broad in comparison to 



