^^6 HUMAN BIOLOGY 



later generations browns and blues appear, with the browns 

 in greater number, as we would expect in a dominant trait. 

 In a hybrid population, derived originally from Nordic and 

 South European stock, we get in the same family brown and 

 blue-eyed children in varying proportions. This is because 

 the germ cells of the parents are dissimilar, due to the hybrid 

 origin of their parents. 



In some other cases, inheritance is more compHcated, 

 as in skin color. It has been shown that in the first generation 

 hybrid between white and a black-skinned negro the children 

 are of an intermediate color, as we see in the mulatto. When 

 two mulattoes are mated their offspring are partly mulatto; 

 sometimes they have a darker color to which the term "Sambo" 

 is apphed; sometimes the lighter color of the quadroon. If, 

 indeed, a large number of the children of such first generation 

 hybrids are examined it will be found that in every sixteen 

 there is, on the average, one white child and one full black. 

 This is the basis for the conclusion that the brown or 

 black skin color depends upon two pairs of factors a and b. 

 In the negro these two pairs are both active in the mulatto; 

 the A and b factors are both present but unpaired. Of the 

 germ ceils of the hybrids some carry the factor a only, some 

 the factor b only, some neither factor and some both factors 

 A and B. When two germ cells, both carrying a and b, unite, 

 the full black color is restored; but when two others unite, 

 neither of which possesses A and b, the offspring will be 

 white. Other combinations will give brown to black skin 

 color. 



Still other traits are even more complicated in their 

 inheritance, partly because there seem to be more than two 

 pairs of factors involved and partly because the development 

 of the trait is to a considerable extent influenced by environ- 

 mental conditions. Thus it is known that a tendency to be 

 over-fat is inherited so that we may speak of an inherited 

 factor in the building of the body. We have reason for 

 believing that slender parents are such by virture of the 

 absence of factors that promote the laying on of fat. In 

 two such slender parents, especially if of slender stock, 

 all of their offspring remain slender. Through excessive 

 feeding or inactivity these offspring may lay on fat but 



