574 GENETICS AND EVOLUTION 



Genes which interact in this fashion have been termed "mutually sup- 

 plementary." 



The inheritance of comb type in poultry provides an interesting 

 example of genie interaction. Leghorns have single combs, Wyandottes 

 have rose combs and Brahmas have pea combs (Fig. 33.3). Each of these 

 types is true breeding. Suitable crosses demonstrate that the gene for 

 rose comb (R) is dominant to single (r) and that the gene for pea comb 

 (P) is also dominant to its allele (p) for single comb. However, when a 

 pea-combed fowl is mated with a rose-combed one, all of the offspring 

 have a different type of comb, resembling half of a shelled walnut and 

 called walnut. When two of these walnut-combed Fi individuals are 

 mated, offspring appear in the ratio of 9 walnut : 3 pea : 3 rose : 1 

 single. We can deduce from this that the genotype of a single-combed 

 fowl must be rrpp; a pea-combed fowl is either PPrr or Pprr; a rose- 

 combed fowl is either ppRR or ppRr, and a walnut comb develops in 

 animals with at least one P and one R gene. Thus the genotypes PPRR, 

 PpRR, PPRr and PpRr all yield walnut combs. Certain Malay varieties of 

 chicken have walnut combs. 



It is clear that there is nothing unusual about the method of in- 

 heritance of any of these genes; the phenotypic ratios observed are simply 

 the result of some variation in the interaction of the genes in the pro- 

 duction of the phenotype. 



281. Multiple Factors 



Many human characteristics, height, body form, intelligence and 

 skin color, and many commercially important characters such as milk 

 production in cows, egg production in hens, the size of fruits, and the 

 like, are not separable into distinct alternate classes, and are not in- 

 herited by single pairs of genes. However, these traits are nonetheless 

 governed by genetic factors; there are several, perhaps many, different 

 pairs of genes which affect the same characteristic. The term multiple 

 factors (or cumulative factors) is applied to two or more independent 

 pairs of genes which affect the same character in the same way and in 

 an additive fashion. When two varieties which differ in some trait con- 

 trolled by multiple factors are crossed, the Fj are very similar to one 

 another and are usually intermediate in the expression of this character 

 between the two parental types. Crossing two F^ individuals yields a 

 widely variable Fo generation, with a few members resembling one 

 grandparent, a few resembling the other grandparent, and the rest show- 

 ing a range of conditions intermediate between the two. 



The inheritance of human skin color was carefully investigated by 

 C. B. Davenport in Jamaica. He concluded that the inheritance of skin 

 color in man is controlled by two pairs of genes, A-a and B-b, in- 

 herited independently. The genes for dark pigmentation, A and B, are 

 incompletely dominant, and the darkness of the skin color is propor- 

 tional to the sum of the dominant genes present. Thus, a full Negro has 

 four dominant genes, AABB, and a white person has four recessive genes, 

 aabb. The Fi offspring of a mating of white and Negro are all AaBb, 



