Independent Segregation 



43 



RY and ry, and the old combinations Ry 

 and rY. So, regardless of how the genes 

 enter the individual, the dihybrid forms four, 

 equally frequent, genetically different gametes. 

 The types and frequencies of gametes 

 formed by the Rr Yy dihybrid can be de- 

 termined more easily after mating it with a 

 double recessive individual, i.e., an individual 

 homozygous recessive for both pairs of genes 

 concerned. In the cross of Rr Yy X rr yy, 

 the double recessive parent produces only ry 

 gametes while the dihybrid produces four 

 different and equally frequent types, RY, Ry, 

 rY, ry. Accordingly, the finding that among 

 the offspring of this cross (Figure 6-6) very 

 nearly 25% are round yellow (55 offspring), 

 25% round green (51 offspring), 25% wrin- 

 kled yellow (49 offspring), and 25% wrinkled 

 green (52 offspring) is a direct confirmation 



both of segregation by the members of a 

 single pair of genes and of independent segre- 

 gation by different pairs of genes. 



Whenever one is dealing with complete 

 dominance, a cross to an individual recessive 

 for the pairs of genes involved will always 

 serve to identify the genotype of the other 

 parent, since the phenotypic types and fre- 

 quencies of the offspring will correspond to 

 the genotypic types and frequencies occurring 

 in the gametes of the latter. This kind of 

 cross is therefore called a test cross, and is 

 also called a hackcross if one of the parents 

 in the series of crosses had been homozygous 

 recessive for the genes under study. 



^™ 



I 



PARENTS Rr Yy x Rr Yy 



OFFSPRING 



% Round 



A Wrinkled 



<; 

 <: 



'A Yellow 9/16 Round Yellow 



'A Green ^ 3/16 Round Green 



'/4 Yellow ^ 3/16 Wrinkled Green 

 Va Green 1/16 Wrinkled Green 



FIGURE 6-5. Phenotypic results of a cross 

 between identical dihybrids. 



\k 



