HEREDITY 1 97 



skin color and behave with all the independence of those determining hue, 

 sleekness, and shagginess of coat in guinea pigs. Each "negro" gene con- 

 tributes a substantial quota of dark brown pigment, while each "white" 

 one produces a small amount of the same or a closely related substance. 

 When the two races cross, these genes combine equally in fertilized 

 eggs, so that mulattoes (the F^ generation) are intermediate in color. 

 Among offspring of the mulatto generation, however, there are bound to 

 be all sorts of combinations. Though the majority will have some black 

 and some white genes, a few will receive only one t)'pe and a few the other. 



Such sorting explains why, after generations of racial crossing, there 

 still are some six hundred and fifty thousand pure negroes in the United 

 States — people without a trace of white color genes. It also lays an old 

 sob story, the one of a white girl who married a man who seemed to be 

 white, only to find that her first baby was "as black as coal." Since all 

 black genes are dominant, a person who seems white almost certainly is 

 SO; many persons whose color genes are as pure as those of their distant 

 white ancestors emerge every year from matings between part-negro 

 parents. Second, no person who is light enough to pass for a white can 

 carry enough genes for negroid color to make his children coal black or 

 even a healthy brown. Each gene produces its quota of pigment, and all 

 blacks are dominant. 



Multiple series of genes controlling one character are very important in 

 man. Several pairs control human height, those for shortness being domi- 

 nant. As a result, two tall parents are likely to have tall children, though 

 some youngsters may violate this rule by receiving all genes for shortness 

 from both father and mother. Short parents, however, are likely to have 

 hidden genes for tallness which combine in their children. 



SOME CHARACTERS ARE LINKED WITH SEX 



Baldness is variously blamed upon disease, worry, hats, mental work, and 

 failure to anoint one's scalp with the latest brand of hair tonic. Instead 

 it is hereditary. Disease can only hurry it a little, while no tonic on earth 

 can prevent it if the right genes are present in a member of the proper sex. 



There actually are three types of baldness, each a distinct character 

 with its own determiners. They are inherited like other genes, but their 

 power to act is controlled by secretions of the sex glands. In males these 

 secretions enable the genes to dominate. Ovarian secretions give the genes 

 so little chance to act that they almost seem to be recessives. 



Common color blindness, which makes red and green look gray, is a 

 sex-linked character, linked to the X chromosome. A man who has received 

 one gene for this trait is color-blind, since his Y chromosome cannot con- 

 tribute the normal dominant. A woman, however, cannot become color- 

 blind unless she inherits the proper recessive genes from both her father 

 and her mother. Since this does not happen very often, few women show 



