Chapter 5, Problem 28P

a. In Drosophila, crosses between F₁ heterozygotes of the form A b / a B always yield the same ratio of phenotypes in the F₂ progeny regardless of the distance between the two genes (assuming complete dominance for both autosomal genes). What is this ratio? Would this also be the case if the F₁ heterozygotes were A B / a b? (Hint: Remember that in Drosophila, recombination does not take place during spermatogenesis.)

b. If you intercrossed F₁ heterozygotes of the form A b / a B in mice, the phenotypic ratio among the F₂ progeny would vary with the map distance between the two genes. Is there a simple way to estimate the map distance based on the frequencies of the F₂ phenotypes, assuming rates of recombination are equal in males and females? Could you estimate map distances in the same way if the mouse F₁ heterozygotes were A B / a b?