no Heredity and Environment 



in color. Hence there seems to be a failure of the Mendelian 

 principle of segregation. 



But white skin is not really white nor is black skin ever perfect- 

 ly black. Davenport has shown that there is a mixture of black, 

 yellow and red pigments in both white and black skins, though the 

 amount of each of these pigments varies greatly in negroes and 

 whites. The relative amounts of these pigments in any given case 

 may be determined by means of a rotating color disk. A white 

 person may have a skin color composed of black (b) 8 per cent, 

 yellow (y) 9 per cent, red (r) 50 per cent, and absence of pigment 

 or white (w) 33 per cent. On the other hand a very black 

 negro may have b 68 per cent, y 2 per cent, r 26 per cent, w 4 

 per cent. The nine children of two mulattoes, the father having 

 13 per cent of black and the mother 45 per cent, ranged all the 

 way from 46 per cent to 6 per cent of black, the latter so far as 

 skin color is concerned being virtually white. On the other hand 

 where both parents have about the same degree of pigmentation 

 the children are more nearly uniform in color; thus seven chil- 

 dren of two mulattoes, the father having 36 per cent and the 

 mother 30 per cent of black, ranged only from 27 per cent to 39 

 per cent of black.* 



Such variations in color in the F 2 and in subsequent genera- 

 tions are exactly what one would expect in a Mendelian character 

 in which more than one factor is involved, as for example in the 

 case of the color of the sweet peas shown in Fig. 33. Davenport, 

 who has made an extensive study of this case, concludes that 

 "there are two double factors (A A, BB) for black pigmentation 

 in the full blooded negro of the west coast of Africa, and these 

 are separably inheritable." These factor^ are lacking in white 

 persons (this being indicated by the formula aa, bb). Since the 

 germ cells carry only single factors and not double ones the cross 

 between negro and white would have only one set of these fac- 



* In another family shown in Fig. 35 the father has 18 per cent black 

 pigment, the mother 38 per cent and the children range from 17 per cent 

 to 54 per cent. 



