CHAPTER 7 



The Interaction of Genes : The Effects 



I . Allelomorphs and Multiple Factors 



In Mendel's original experiments, the two allelomorphic forms of 

 the genes used were related to one another as dominant and recessive. 

 That is, one of the pair, the dominant, came to complete expression in 

 the heterozygote, suppressing all signs of the other recessive gene. 

 This relationship is not a necessary one; all gradations are known 

 between cases in which the heterozygote shows the full effect of one 

 gene (complete dominance) and cases in which the heterozygote is 

 strictly intermediate and it is impossible to speak of either gene as 

 dominant. A considerable degree of dominance is, however, much 

 more usual than would be expected on a basis of pure chance, and 

 several attempts have been made to explain why this should be; they 

 are considered later (p. 297). 



The relation of dominance-recessiveness depends on the interaction 

 between two allelomorphic genes, but interactions of a similar kind 

 exist between genes belonging to different loci; in fact, all the genes in 

 the genotype probably react with one another during development. 

 We shall discuss the nature of the reactions in the next chapter: 

 probably they are always reactions between gene-products rather than 

 between genes themselves. Here we shall examine the evidence that 

 such interactions do occur. 



One form of this interaction is the dependence of a phenotypic 

 character on the presence of two different genes. A well-known case is 

 the walnut comb of fowls, which is the form assumed in the presence of 

 both of two factors R and P, of which R alone gives the rose comb and 

 P alone the pea comb, both of which are dominant over r ot p which 

 give the simple "single" comb. Another well-known example was 

 found by Bateson and Punnett in sweet-peas, where the formation of 

 coloured flowers is dependent on the presence of two dominant factors 

 C and P. 



The term complementary factors is usually reserved for cases in 

 which a simple presence or absence of a character is determined by the 

 presence or absence of two or more specific members of different loci. 

 Many more cases are known in which the degree or kind of character 



