GENETICS 575 



with two dominant genes and a skin color (mulatto) intermediate be- 

 tween white and Negro. The mating of two such mulattoes produces 

 offspring with skin colors ranging from full Negro to white (Table 11). 

 A mulatto with the genotype AaBb produces four kinds of eggs or sperm 

 with respect to the genes for skin color: AB, aS, Ab and ab. From a 

 Punnett square for the mating of two doubly heterozygous mulattoes 

 (AaBb) it will be evident that there are 16 possible zygote combinations: 

 one with four dominants (black), four with three dominants (dark 

 brown skin), six with two dominants (mulatto), four with one dominant 

 (light brown skin) and one with no dominants (white skin). The genes 

 A and B produce about the same amount of pigmentation and the geno- 

 types AaBb, AAbb and aaBB produce the same phenotype, mulatto skin 

 color. 



This example of multiple factor inheritance is fairly simple, for 

 only two pairs of genes appear to be involved. With a larger number 

 of pairs of genes, perhaps ten or more, there are so many classes, and 

 the differences between them are so slight, that the classes are not dis- 

 tinguishable. A continuous series is obtained. The inheritance of human 

 stature is governed by a large number of pairs of multiple factors, with 

 shortness dominant to tallness. Since height is affected not only by these 

 multiple factors but also by a variety of environmental agents, there are 

 adults of every height from perhaps 55 inches up to 84 inches. If we 

 measure the height of 1000 adult men selected at random and draw a 

 graph of the number having each height, we will obtain a bell-shaped 

 normal curve, or curve of normal distribution (Fig. 33.4). It is evident 

 that there are few extremely tall or extremely short men, but many of 

 intermediate height. This resembles the Fg of the simpler situation with 

 skin color, for there were few individuals with black or white skin but 

 many with mulatto skin. 



All living things show comparable variations in certain of their 

 characteristics. If one were to measure the length of 1000 shells from the 

 same species of clam, or the weight of 1000 hen's eggs, or the amount of 

 milk produced per year by 1000 dairy cows, or the intelligence quotient 

 (I.Q.) of 1000 grade school children, and make graphs of the number of 

 individuals in each subclass, one would obtain a normal curve of dis- 

 tribution in each instance. The variation is due in part to the action 

 of multiple factors and in part to the effects of a variety of environ- 

 mental agents. In a few species it has been possible to establish strains 

 which are genetically identical-all the individuals have exactly the 



Table 11. MULTIPLE FACTOR INHERITANCE OF SKIN COLOR IN MAN 



Parents AaBb AaBb 



(Mulatto) (Mulatto) 



Gametes AB Ab aB ab AB Ab aB ab 



Offspring: 



1 with 4 dominants— AABB— phenotypically Negro 



4 with 3 dominants— 2 AaBB and 2 AABb— phenotypically "dark" 



6 with 2 dominants— 4 AaBb, 1 AAbb, 1 aaBB— phenotypically mulatto 



4 with 1 dominant— 2 Aabb, 2 aaBb— phenotypically "light" 



1 with no dominants— aabb— phenotypically white 



