THE PRINCIPLES OF HEREDITY 515 



dominant-recessive or intermediate inheritance. We know, for instance, 

 that height in man is influenced by heredity ; yet we do not have just 

 tall people and short people as would he the case if it were simple 

 dominant-recessive inheritance. Neither could we assume intermediate 

 inheritance from the influence of a single gene, for body height does 

 not fall into three sharply defined classes. There are many gradations 

 of height. Of course, environment in the form of food, disease, etc., 

 plays its part, but the genes determine the basic potentialities of growth. 

 To understand the method of inheritance of such quantitative character- 

 istics does not require a study of a new type of gene or a new method 

 of inheritance — it is simply explained on the basis of multiple genes. 



Body height in man results from a complicated interaction of a 

 number of different factors. If a person receives a good proportion of 

 the genes which favor extended skeletal growth, he will be above 

 average in height ; and vice versa if more genes for short stature are 

 received. Through independent segregation of the genes in the re- 

 productive cells of his parents a brother or sister may get a different 

 proportion of the two and show a marked difference in height. Fig. 

 32.8 shows how such segregation can operate to produce a tall child 

 from parents of medium height. In general, however, the average 

 adult height of all the children of a couple will approximate the parent 

 height, although it may run a little greater due to the better environ- 

 mental conditions which the children of the present generation have 

 enjoyed. Also, in any comparisons of height in human beings it is 

 important to keep the influence of sex in mind. Because of the earlier 

 physical maturity of the female, a woman's adult height will be several 

 inches less than it would have been had she been a man. To convert 

 a woman's height into a figure that will compare with that of a man 

 it is necessary to multiply her height by 1.08. 



The inheritance of skin color in man is another case which illustrates 

 the influence of more than one gene on a characteristic. There seem 

 to be two pairs of genes involved in the skin color differences between 

 the Negro and the white races. Thus, a white person would carry four 

 genes for the white-type skin color and the Negro would carry four for 

 the Negro-type pigmentation. When there is an interracial marriage, 

 mulatto children are produced which have an intermediate type of skin 

 color. Each of these children will receive two genes of each type, and 

 both are intermediate in their expression. Among the children of two 

 mulattoes, on the other hand, the genes become independently assorted 

 to give five different classes of skin coloring, ranging from white to the 

 Negro type. If this is worked out as a dihybrid cross the ratio of the 

 shades of skin color will be 1:4:6:4:1. There are certainly other genes 



