A dominant allele H reduces the number of bristles that Drosophila flies have, g

Question

A dominant allele H reduces the number of bristles that Drosophila flies have, giving rise to a "hairless" phenotype. In the homozygous condition, H is lethal. An independently assorting dominant allele S has no effect on bristle number except in thje presence of H in which case a single dose of S supressess the hairless phenotype, thus restoring the hairy phenotype. However, S also is lethal in the homozygous (S/S) condition

a. What ratio of hairy to harless flies would you find in the live progeny of a cross between two hairy flies both carrying H in the supressed condition?

b. When the hairless progeny are backcrossed with a parental hairy fly, what phenotypic ratio would you expect to find among their live progeny?

Explanation / Answer

a) Since they are hairy and contain H their genotypes are both HhSs HhSs x HhSs yields: 4 HhSs - hairy 2 Hhss - hairless 2 hhSs - hairy 1 hhss - hairy The rest (27 out of 36) are dead due to homozygosity at either locus Therefore phenotypic ratio of hairy:hairless = 7:2 b) Hhss x HhSs 4 HhSs - hairy 4 Hhss - hairless 2 hhSs - hairy 2 hhss - hairy The rest (24 out of 36) are dead. phenotypic ratio of hairy:hairless = 8:4 or 2:1

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