Phenomena of Inheritance 



109 



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 factors are lacking in white 

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



c? AB Ab aB 



a b 



AB 



Ab 



aB 



a b 



FIG. 34. CHECKERBOARD DIAGRAM SHOWING RESULTS OF CROSSING Two 

 MULATTOES, each having color factors ABab. Types of male germ cells 

 are above the square, of female cells on the left and the possible combina- 

 tions of these are shown in the 16 small squares. Homozygotes are found 

 only along the diagonal. The color of the children varies all the 

 way from black (upper left corner) to white (lower right corner). 



* In another family shown in Fig. 36 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. 



