50 



CHAPTER 4 



give rise to the 9:3:3: I phenotypic ratio in 

 crosses between identically dihybrid garden 



peas'.' I'wo factors were responsible. One 

 was the occurrence of dominance for each 



pair oi alleles; this converted the 1:2:1 

 genotypic ratio for each gene pair to a 3:1 

 phenotypic ratio. The other factor was that 

 the action of the two gene pairs was inde- 

 pendent and resulted in detectable effects on 

 different traits. This permitted both 3:1 

 ratios to be recognized separately even when 

 these ratios were distributed at random in 

 the progeny (Figure 4-5). It becomes ap- 

 parent, therefore, that the particular pheno- 

 typic ratios obtained, following crosses in- 

 volving more than one gene pair, depend 

 both upon the dominance relationships be- 

 tween alleles and the gene interaction rela- 

 tionships between nonalleles. 



If neither gene pair shows dominance, and 

 if each pair acts both independently and on 

 different traits, two 1:2:1 phenotypic ratios 

 will be produced, and these, when distributed 

 at random, will result in the 1:2:1:2:4:2: 

 1:2:1 phenotypic ratio. Here, because no 



genotype is masked phenotypically by any 

 other, the phenotypic and genotypic ratios 

 are the same. (This would also be true of 

 the following crosses: A A' BB' X AA BB, 

 AA' BB' X A' A' B'B', AA' BB X AA BB'. ) 

 This kind o\ result is illustrated in the prog- 

 eny of parents both of whom have thalas- 

 semia minor (77) and MN (MM') "blood 

 type." (MM is phenotypically M, M'M' is 

 phenotypically N, as described on p. 58.) 



When, however, the aforementioned con- 

 ditions are changed so that one of the two 

 pairs of genes shows dominance, two dif- 

 ferent genotypes will produce the same 

 phenotype, and fewer than 9 phenotypes are 

 expected. Thus, if B is dominant to B\ 

 genotypes 1 and 2 (in Figure 4-9) are ex- 

 pressed as one phenotype, genotypes 4 and 

 5 as another, and 7 and 8 as another, so that 

 the phenotypic ratio becomes 3:1:6:2:3:1. 

 This is the phenotypic ratio expected in the 

 progeny of parents both heterozygous for 

 albinism (Aa) and having MN blood type 

 (MM'). If both gene pairs show domi- 

 nance, one phenotype is expressed by geno- 



FIGURE 4-10. Drosophila melanogaster itui- 

 tants showing the no-wing (left) and the curled 

 wim; ( right ) phenotypes. ( Drawn by E. M. 

 Wallace.) 



