598 



CONTINUITY OF LIFE 



4 albino. Here we see two independent 

 gene pairs interacting in such a way that 

 one dominant will produce its effect with or 

 witliout the other. The second dominant, 

 however, can produce its effect only in the 

 presence of the first. 



Multiple factors 



So far we have been considering char- 

 acters that are clearly defined, where the 

 phenotypes are sharply set off by color or 

 some other trait. But there are a great many 

 characters which, instead of demonstrat- 

 ing clear-cut differences, show continuous 

 variation from the two extremes; this is 

 particularly true of such human traits as 

 skin color, height, weight, special abilities, 

 and intelligence. In cross-matings between 

 individuals possessing the two extremes of 

 these traits, children do not resemble one 

 parent or the other but are intermediate 

 between the two. This is like blending in- 

 heritance in which more than one pair of 

 genes is involved. This means that several 

 genes, known as multiple factors, influence 

 one trait and their ultimate effect depends 

 on additive action. This can be illustrated 

 by a consideration of skin color in humans, 

 which is the best-known example of this 

 type of inheritance. 



In certain parts of the world where there 

 is no "color line," marriages between Ne- 

 groes and whites are common, and such 

 practice has afforded geneticists sufficient 

 information about the inheritance of skin 

 pigmentation to understand it genetically. 

 It is known that two pairs of genes on sep- 

 arate chromosomes are involved. These 

 may be called Aa and Bh where A and B 

 are responsible for pigmentation. In the Fi 

 generation resulting from a mating between 

 a homozygous Negro ( kKBB ) and a homo- 

 zygous white (aabb) we would expect the 

 hybrid ( AaBb ) to be either all black if the 

 genes for pigmentation were completely 

 dominant or half black, mulatto, if blend- 

 ing occurred. The latter proves to be the 



case. The genes for pigmentation show in- 

 complete dominance and the offspring are 

 dark-skinned, approximately intermediate 

 between the white and black parents. This 

 resembles exactly what one would expect 

 in a single gene pair trait. The double gene- 

 pair proof comes when two mulattos mate 

 (AaBb X AaBb). If there were but a single 

 gene pair, the typical 1 white : 2 mulatto : 

 1 black ratio would be expected, but in- 

 stead an entirely new ratio appears, that is, 

 1 black : 4 dark brown : 6 mulatto : 4 light 

 brown : 1 white. Since there are sixteen 

 possibilities, there must have been four 

 genetically different sperms and four ge- 

 netically different eggs, or two pairs of 

 genes, each located in different chromo- 

 somes. This is simply the Fo generation of 

 a dihybrid cross. Why, then, should this 

 peculiar ratio result instead of the typical 

 9:3:3:1? 



If we assume that the genes A and B 

 influence the production of equal amounts 

 of pigmentation even though located on 

 different chromosomes, and that blending 

 occurs between each with its recessive, a 

 and b, then the 1:4:6:4:1 ratio is exactly as 

 one would expect. Moreover, the intensity 

 of the skin pigmentation, that is, the pheno- 

 type, would give a clue as to the genetic 

 constitution or genotype of an individual. 

 The darker the skin, the more capital let- 

 ters would appear in the genie constitution. 

 For example, a full Negro would have the 

 genetic formula of AABB; a dark brown 

 Negro would be AABb or AaBB; a mulatto 

 would, of course, have one of each gene, 

 AaBb. However, it would be impossible to 

 tell whether the formula was AAbb, AaBb 

 or aaBB, since each dominant gene has the 

 same influence in producing pigmentation. 

 The light brown would have but one 

 dominant gene, Aahb or aaBb, whereas the 

 white would be double recessive {aabb), 

 naturally. Thus, in this Fo generation there 

 would be one person resembling each of 

 the grandparents and all others would be 

 graded in respect to pigmentation. 



