Question

An eyeless female fly is crossed to a male fly with normal eyes. Among the progeny,
the males all lack eyes, and the females all have shrunken heart-shaped eyes. Explain the
underlying genotypes that cause this to happen.

Answer 1

Because the phenotypes differ depending on the sex of the offspring, we suspect that the trait is probably X linked.

You know that the mother fly is eyeless, which means that both her X chromosomes have the mutant "no eyes" allele. All her offspring get one of these two alleles (doesn't matter which one, because they're both the same). The male offspring do not get another X - instead, they get a Y from their father which makes them male. Because they do not have a functioning copy of the allele, they are all eyeless. The female offspring do get another X chromosome from their Dad and this one has a normal, wild type allele for eyes. Because the females have shrunken eyes (as opposed to normal ones), we know the trait is incompletely dominant, meaning that heterozygotes express the recessive phenotype a little.

Clear?

Answer 2

Very clear :) thank you again!

Answer 3

You're welcome. :D