196 READINGS IN BIOLOGICAL SCIENCE 



and at least one kind of dwarf appears as a simple Mendelian recessive. 

 Short fingers are produced by a dominant gene which also makes the whole 

 body short and stocky. Those of us who lack these defects are "pure" for 

 the normal recessive gene, which lets bones, especially those of the fingers, 

 develop to full length. 



There seems no doubt that human albinism, like that of guinea pigs, is 

 a recessive character that appears only when a fertilized egg receives two 

 appropriate genes. One gene has no effect, for it is hidden by normal color- 

 ing of skin, eyes, and hair. Since albinos frequently do not marry, this re- 

 cessive character affects only about one person in ten thousand, or a total 

 of about thirteen thousand people in the entire United States. On the other 

 hand, a white patch of hair above the forehead is caused by one or two 

 dominant genes whose action seems to be regulated by age, and perhaps by 

 other factors. As a result, the character sometimes appears in small children, 

 but may be delayed until those who possess it are twenty or thirty years 

 old. 



HAIR COLOR INVOLVES SEVERAL GENES 



Since World War I we have heard a great deal about the Nordics, a 

 tall and supposedly superior race whose narrow heads are covered with 

 yellowish or straw-colored hair, which may be so pale as to look almost 

 white. Studies of heredity show that these blond hues depend upon one pair 

 of recessive genes which seemingly appeared as mutants and replaced 

 darker genes among folk who wandered into northern Europe some five 

 thousand years ago. These genes combined with other recessives control- 

 ling great height, fair skin that does not tan, and blue eyes. 



After becoming racially distinct the Nordics spread far and wide through 

 western Europe, interbreeding with other peoples whom they met or con- 

 quered. Nordics grew darker and darker as they intermarried until their 

 racial traits survived only in shape of skull and other obscure characters. 

 But their genes for hair color did not vanish. Today the heredity of Eu- 

 ropean races is as mixed in this respect as it is in most other characters. 



GENES MAY CO-OPERATE 



Skin color, which differs from the power to make sun-tan pigment, is 

 complicated. When whites and pure negroes cross, their children are mu- 

 lattoes of intermediate type. This suggests partial dominance, but the chil- 

 dren of mulattoes are too varied for any 1:2:1 ratio. Indeed, they show 

 almost every conceivable gradation between the pale skin of one grand- 

 parent and the "black" one of the other. 



These gradations have long puzzled scientists, especially of the United 

 States, where racial color is the subject of strong prejudice. After col- 

 lecting an enormous number of records these investigators conclude that 

 both whites and negroes have four pairs of contrasting genes that control 



