Question

An allele in Drosophila produces a star-eye trait in the heterozygous individual. However, the star-eye allele is lethal in homozygotes. What would be the ratio of phenotypes of surviving offspring if star-eyed flies were crossed to each other?

Short answer

The ratio of phenotypes of surviving offspring when star-eyed flies are crossed to each other is 2:1 (star-eyed:normal).

Step-by-step solution

Identify the genotypes of the parents

The parent flies have the star-eye trait, indicating they carry the star-eye allele. Because homozygotes are lethal, we must assume these flies are heterozygous meaning they carry one star-eye allele (S) and one normal allele (s). Therefore, the genotype of both parents is Ss.

Construct a Punnett square

A Punnett square can be used to predict the genotypes of offspring from a genetic cross. Construct a 2x2 Punnett square with one parent's alleles on top and the other's on the side.

Fill in the Punnett square

Fill in the Punnett square from step 2 with the possible genotypes of the offspring. Each square is filled by combining one allele from the top with one from the side. This results in the genotypes SS, Ss, sS, ss.

Determine surviving offspring

Now, remember that homozygous for the star-eye allele (SS) is lethal. So, these flies will not survive, leaving only offspring with Ss and ss genotypes.

Calculate the ratio of surviving offspring

Lastly, calculate the ratio of surviving offspring genotypes. Count the number of each type (Ss and ss) and express the result as a ratio. This gives a 2:1 ratio of star-eyed to normal-eyed flies among the surviving offspring.