Suppose a and b are recessive alleles of genes located 20 map units apart on chr

Question

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

Explanation / Answer

Answer:

F1 X F1

a +/+b (male) X a +/+b (female)

Due to no recombination, only two types of gametes are produced. Those are a+ & +b.

Distance between the genes = % Recombination frequency.

Females produce the following gametes

Parental gametes (a + & + b )= 80% (each 40%)

Non parental gametes(+ + & a b) = 20% (each 10%)

a + (0.4) + b (0.4) + + (0.1) a b (0.1) a + (0.5) a +/a + (0.2%) a +/+ b(0.2%) a +/+ + (0.05%) a +/a b (0.05%) + b(0.5) a +/ + b (0.2%) + b/ + b (0.2%) + b/+ + (0.05% + b /a b (0.05%)

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