Chapter 20, Problem 29P

In Drosophila, recessive mutations in the fruitless gene (fru) result in males courting other males; and recessive mutations in the Antennapedia gene ( ${\text{Ant}}^{-}$) lead to defects in the body plan, specifically in the thoracic region of the body, where mutants fail to develop legs. The two genes map $15$ cM apart on chromosome $3$. You have isolated a new dominant ${\text{Ant}}^d$ mutant allele that you induced by treating your flies with X $-$ rays. Your new mutant has legs developing instead of antennae on the head of the fly. You cross your newly induced dominant ${\text{Ant}}^d$ mutant (a pure $-$ breeding line) with a homozygous recessive fru mutant (which is homozygous wild type at the ${\text{Ant}}^{+}$ locus), as diagrammed below:

$\frac{{\text{Ant}}^{\text{d}}{\text{fru}}^{\text{+}}}{{\text{Ant}}^{\text{d}}{\text{fru}}^{\text{+}}}\text{×}\frac{{\text{Ant}}^{\text{+}}\text{fru}}{{\text{Ant}}^{\text{+}}\text{fru}}\to {\text{F}}_{\text{1}}\frac{{\text{Ant}}^{\text{d}}{\text{fru}}^{\text{+}}}{{\text{Ant}}^{\text{+}}\text{fru}}$

a. What phenotypes, and in what proportions, do you expect in the ${\text{F}}_2$ obtained by interbreeding ${\text{F}}_1$ animals?

b. Your cross results in the following phenotypic proportions:

Legs on head, normal courting behaviour $75$

Normal head, abnormal courting behaviour $25$

Legs on head, abnormal courting behaviour $0$

Normal head, normal courting behaviour $0$

Provide a genetic explanation for these result and describe a test for your hypothesis.

c. Provide a molecular explanation for the reason your new ${\text{Ant}}^d$ mutant is dominant and for its novel phenotype.