io8 Heredity and Environment 



grained and white-grained wheat he usually got in the second gen- 

 eration the monohybrid ratio of 3 red to I white, but three strains 

 gave the dihybrid ratio of 15 to I and two gave the trihybrid ratio 

 of 63 to i. Consequently he concludes that while the red color of 

 wheat grains is usually due to one factor for red, it may in some 

 cases be due to two or even to three factors ; notable departures 

 from expected ratios may thus be explained. Other departures 

 from regular Mendelian ratios are caused by the early death of 

 certain gametes or zygotes due to lethal factors, as explained on 

 page 104. 



Blending of Color in Mulatto. Perhaps the most serious objec- 

 tions which can be presented against the universality of the Men- 

 delian doctrine are found in phenomena of "blending" inheritance. 

 In some instances contrasting characters of parents appear to blend 

 in offspring and even in the F 2 and in subsequent generations the 

 descendants remain more or less intermediate between the parents. 

 One of the best known illustrations of this is found in the skin col- 

 or of the mulatto which is intermediate between the white parent 

 and the black one, and even in the F 2 and in subsequent generations 

 mulattoes do not usually produce pure white or pure black chil- 

 dren, though the children of mulattoes show considerable variation 

 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 



