306 Cell as Basis of Organic Activity 



In fact all combinations totaling 16 in the F^ may result by inter- 

 action of two factors. Some ratios are 12:3:1, 13:3, 9:3:4; all totaling 

 16, but occurring in different combinations. 



PARENTS 



c::> 



BLACK ALBINO 



CCaa ccAA 

 Ca cA 



Fig. 95. — Another cross in which two allelomorphic pairs interact to control a single, 

 phenotypic character, in this case coat color in guinea pigs. 



Skin color in human beings illustrates the condition of cumulative 

 factors. In this case, two pairs of allelomorphic genes are again in- 

 volved. In this cross, the more dominants present, the darker the skin. 

 Black has four dominants represented thus: AABB; while white is 

 aahh. The mulatto is AaBh, aaBB, or AAbb ; dark is AABb or AaBB, 

 while light is aaBb or Aabb. 



Fig. 96. — A cross involving cumulative factors. In skin color in human beings, 4 

 dominants produce black; 3 produce dark; 2, mulatto; 1, light; and none, white. 



