190 GENETICS 



10. HUMAN SKIN COLOR 



Finally, in man the skin-color of mulattoes, in hy- 

 brids between blacks and whites, has often been men- 

 tioned as a case of blending inheritance since mulattoes 

 are commonly supposed to produce mulattoes when 

 they mate together or a blending degree of color when 

 they mate with some one whose shade of color is unlike 

 their own. 



This matter has been carefully and extensively 

 studied by Davenport and Danielson l who came to the 

 conclusion that the pure-blooded negro of the West 

 Coast of Africa possesses two pairs of duplicate genes 

 for black pigmentation (A ABB) which, though sepa- 

 rately heritable, are cumulative in effect. The corre- 

 sponding formula for black pigmentation in a normal 

 white is adbb. When black (AABB) and white (aabb) 

 are crossed, the formula for the mulatto will be AaBb 

 in which half the total amount of black pigment of both 

 parents is present. 



The result of crossing two mulattoes is shown by 

 checkerboard diagram in Fig. 42. 



The figures in the corners of the squares indicate 

 the total amount of black pigment in each case upon 

 the supposition that A = 19, B = 1 6, # = 2 and 

 b = 1, these values being determined by the color-top 

 method described by Davenport and Danielson. 



In the table on page 191 is a classification of these 

 possibilities according to the amount of black pigment 



1 Heredity of Skin Color in Negro and White Crosses. Pub. 

 No. 188 of the Carnegie Inst. of Washington, 



