362 



EVOLUTION AND GENETICS 



Pigmentation of Skin. Davenport's studies of negro-white 

 crosses are the chief source of information on this subject. He 

 reached the conclusion that pigmentation depends on two pairs 

 of cumulative factors which may be designated as AA and BB 

 (Fig. 199). Since the white race is not totally devoid of pigment 



AB Ab aB ab 



AB 



Ab 



aB 



a b 



Fig. 199. — Punnett square showing the expected shades of color in the pos- 

 sible offspring of two mulattoes. A = 18, B = 17, a = 2, and 6 = 1 per cent of 

 black pigment. (From Walter, after data from Davenport and Danielson.) 



it follows that the factors aa and l)b do not stand for albinism 

 but only for slight pigmentation. The percentage value ascribed 

 to each determiner by Davenport is indicated in the diagram. 

 A pure African l^lack would have the formula AABB for color, 

 while a white would be represented by aabb. The hybrid mulatto 

 has the formula AaBb. Figure 199 represents the F2 generation 

 derived from mulatto parents. In this diagram it is evident that 

 there are three kinds of mulattoes with the formulae AAbb, AaBb 

 and aaBB, each differing slightly from the others in pigmentation, 

 as well as intergrades between these and the dominant and 

 recessive combinations. The names for these intermediates are 

 quadroon for individuals with only one dominant determiner and 



