Sometimes one gene may have the effect of suppressing an abnormal (mutant) allel

Question

Sometimes one gene may have the effect of suppressing an abnormal (mutant) allele of another gene, resulting in the normal phenotype. Flies that are homozygous for the mutant gene, suppressor or hairy wing (s/s) will have a normal phenotype if they are also homozygous for the mutant gene, hairy wing (h/h). The following questions refer to the offspring ratios after the mating of individuals heterozygous for the wild type and mutant alleles of BOTH suppressor of hairy wing (s+/s) and hairy wing (h+/h) genes. The genes, s+ and h+ are located on separate chromosomes. Use the symbols sD, sR, hD, and hR to denote the genotype of a fly that is heterozygous for:

A. Dominant (D) mutations in both s+ and h+

B. Recessive (R) mutations in both s+ and h+

i. What phenotypic ratio of offspring would you expect if suppression occurs when you sib-mate flies heterozygous for dominant mutations in both s+ and h+ ?

ii. What phenotypic ratio of offspring would you expect if suppression occurs when you sib-mate flies heterozygous for recessive mutations in both s+ and h+ ?

iii. Draw the punnett squares showing both the genotype and corresponding phenotype for EACH of the 16 outcomes for both scenarios.

Explanation / Answer

1. All the 4 offspring will have phenotypic ratio as hairy wing because of the dominant mutation.

2.All the 4 offspring will have phenotypic ratio as hairy wing because of the recessive mutation.

3.

Ds+ Ds+ Dh+ Dh+s+ Dh+s+ Dh+ Dh+s+ Dh+s+ Rs+ Rs+ Rh+ Rh+s+ Rh+s+ Rh+ Rh+s+ Rh+s+ Dh+s+ Dh+s+ Dh+s+ Dh+h+s+s+ Dh+h+s+s+ Dh+s+ Dh+h+s+s+ Dh+h+s+s+ Rh+s+ Rh+s+ Rh+s+ Rh+h+s+s+ Rh+h+s+s+ Rh+s+ Rh+h+s+s+ Rh+h+s+s+

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