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 a + phenotype?

0%

5%

10%

25%

50%

Solution

Answer is 0%

P generation

True breeding a b male X true breeding + + female

Gametes a , b x + , +

   |

   a b + + F ₁ generation

   | selfing

gametes a, b ++ a,b ++

a b++ gametes

F _{2 generation}

F_{2 phenotypes are the following}

aa-1

ab++ - 2

b++ b++ - 1

a+ phenotype = 0%