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 + males are mated to true-breeding + b females. In the F1 generation the heterozygous a +/+ b flies are crossed to each other. What proportion of F2 flies exhibit the recessive a phenotype and dominant + phenotype for b in the next generation (i.e., are like the a + males of the P generation)?

You may wish to use a Punnet square again, but remember, there is no recombination in Drosophila males.

Solution

Given - :

a and b are 20units apart it means the recombinant percentage is 20 percent.

Also a+ males are mated with +b females, so in first generation we get

F1 generation

aa+ x +bb =ab,a+b, a++b, a+b

F2 generation

a+b x a+b=a+a, a+b, +ba, b+b

Out of these 1/4 represent the trait +ba for recessive a phenotype and dominant + phenotype for b in F2 generation.

So the 25% of F2 flies represent the following above trait.