Chapter 7 



GENE INTERACTION 



AND PHENOTYPIC EXPRESSION 



Yi 



"ou ARE already familiar with 

 some of the phenotypic con- 

 sequences of gene interaction, 

 in that the phenotype of a heterozygote may 

 show the effects of only one allele, or some 

 of the effects of both alleles, or the complete 

 effects of both alleles. These phenotypic 

 consequences have already been called com- 

 plete, partial, and no dominance, respec- 

 tively. In the garden pea hybrids already 

 discussed, complete dominance was respon- 

 sible for the 3:1 phenotypic ratio obtained 

 from crossing two monohybrids. This 

 necessitated the extra labor of testing the off- 

 spring possessing the dominant phenotype 

 in order to identify the 1:2:1 genotypic 

 ratio predicted from such crosses. Had no 

 dominance obtained the phenotypic ratio 

 would have been the same as the genotypic 

 one. Nevertheless, in all cases genes were 

 segregating, and the specific ratios observed 

 depended only upon the dominance relation 

 within the gene pair, that is, the relation 

 between the expression of one allele and that 

 of its partner. 



You have seen also that complete domi- 

 nance had no influence upon the indepen- 

 dence of the segregation of different pairs of 

 genes within a given individual. The geno- 

 typic ratio expected from crossing two par- 

 ticular dihybrids has already been derived 

 (Chapter 6). Let us rederive this ratio, em- 

 ploying more general symbols for genes, for 

 reasons soon to be apparent, using the 

 branching track method in a still slightly 



different way. Let A and A' be one pair 

 of alleles and B and B' another. Mating 

 AA' BB' by AA' BB' will give the genotypic 

 ratio shown in Figure 7-1. 



Note that among every 16 offspring, on the 

 average, there would be 9 different genotypes: 

 1 with all unprimed gene symbols, 1 with all 

 primed gene symbols, and 7 others having 3 

 primed or 3 unprimed or 2 primed gene 

 symbols. Let us re-examine how this geno- 

 typic ratio gave rise to the 9 : 3 : 3 : 1 pheno- 

 typic ratio in crosses between dihybrid garden 

 peas. Two factors were responsible. One 

 was the occurrence of dominance within each 

 pair of alleles, the other was the fact that the 

 trait determined by one pair of genes was un- 

 associated with the trait determined by the 

 other pair of genes. In what way does the 

 phenotypic ratio expected from crosses be- 

 tween dihybrids for unrelated traits depend 

 upon whether dominance obtains for neither, 

 one, or both pairs of genes? This can be 

 answered with the aid of the left side of 

 Figure 7-2. 



In any column in Figure 7-2, boxes filled 

 with the same symbol have identical pheno- 

 types. Note that in DI the genotypic and 

 phenotypic ratios are identical, and that the 

 other D columns have two or more geno- 

 types represented by a single phenotype. DI 

 could be exemplified by the phenotypic ex- 

 pectation from matings between two people 

 both having AB and MN blood types (see 

 Chapter 4). DII could be exemplified by the 

 phenotypic expectation from matings be- 

 tween two people both of MN blood type 

 and heterozygous for albinism (see Chapter 

 4). We have already discussed Dili in 

 Chapter 6. 



It should be recalled that the genes for 

 round-wrinkled and the genes for yellow- 

 green though affecting the same part of a pea 

 plant, the seed, act on different traits — tex- 

 ture and color, there being no obvious rela- 

 tionship between the two. So in this and 

 the other cases under D a particular part 



