CHAPTER 7 



AA' X AA' BB' X BB' 



AA'BB' X AA'BB' x 1/16 



y4AA 



% AA' 



74 A'A' 









1 AABB 



2 AABB' 



1 AAB'B' 



2 AA'BB 

 4 AA'BB' 

 2 AA'B'B' 



1 A'A'BB 



2 A'A' BB' piGURE 7-1. 



Recombination 



1 A'A' B'B' frequencies. 



of an organism is capable of showing the 

 presence of any phenotype due to one pair 

 of genes at the same time as it shows any 

 phenotype of another pair. Under D, then, 

 the two pairs of genes produce effects which 

 are independently distinguishable because 

 they do not impose upon each other's ex- 

 pression, i.e., they do not superpose. 



What phenotypic ratios are expected when 

 the two different pairs of genes affect the 

 same trait in the same direction (Figure 7-2, 

 S)? The ratio in SI would follow if any un- 

 primed gene contributed an equal and cumu- 

 lative effect on the phenotype, say by forming 

 melanin pigment, the primed genes con- 

 tributing none of this effect. SII would fol- 

 low for cumulative effects when A A, AA' , and 

 BB' each produces equal phenotypic effect, 

 say on height, BB produces twice this effect, 



and A'A' and B'B' produces none of this 

 effect. SIII would follow if either A 01 B 

 gives the full phenotypic effect, say on flower 

 color, only A'A' B'B' producing none of this 

 effect. 



In each of the examples under S the ratios 

 obtained were simplifications of the corres- 

 ponding ratios found under D, due to the 

 fact that different combinations of alleles from 

 two different pairs of genes acting in the same 

 direction gave the same phenotypic effect. 

 In these cases, then, different pairs of genes 

 have a common phenotypic background on 

 which their effects superpose, the effect of one 

 gene interfering with the detection of the effect 

 of the other pair. 



What may the phenotypic ratios from 

 crosses between identical dihybrids become, 

 when both pairs of genes show dominance 



