Suppose a and b are recessive alleles of genes located 20 ma

Suppose a and b are recessive alleles of genes located 20 map units apart on chromosome 2 in Drosophila melanogaster. For the P generation, true-breeding a b males are mated to true-breeding + + females. In the F1 generation the heterozygous a b/+ + flies are crossed to each other. What proportion of F2 flies exhibit the recessive a b phenotype (i.e., are like the males of the P generation)?

Solution

Answer:

a b/+ + plant produces 4 types of gametes in the following percentes.

ab & ++ gametes (parental) 80% (each 40%)

a+ & b + gametes (recombinant)20% (each 10%)

ab/ab = 40% * 40% = 16%

Both recessive(ab/ab) progeny produced in the dihybrid cross is 1.

The proportion of recessive ab phenotype = 1* 16% = 16% or 0.16