The fruit fly Drosophila has 8 chromosomes (2n = 8). This includes one pair of s

Question

The fruit fly Drosophila has 8 chromosomes (2n = 8). This includes one pair of sex chromosomes (XX in females and XY in males). The other three pairs of chromosomes are autosomes. I give you a vial of flies with two dominant mutations: the first one Stubble, Sb, makes the bristles short and stubby (wild type bristles are longer and pointed; Sb+), whereas the other one, Serrate, Ser, makes the edge of the wing serrated (wild type wings have a smooth, not serrated margin Ser+). The genes encoding both these traits are on chromosome 3.

I give you a vial of flies that carry both these dominant mutations. Set up a cross to determine if the two mutations are in cis (that means that the mutant alleles are on the same homolog) or if they are in trans. Write out the cross along with the possible outcomes. Consider one possibility at a time.

Explanation / Answer

I don't really have a good answer for this question, and now I'm trying to figure it out too. We have two different mutations (serrated wing and stubby hairs), located on different genes (different loci) but same chromosome. The genes can be cis, meaning on the same homolog, or trans, meaning on opposite homologs. In the cis case, the stubble gene Sb is on one homolog, then at the same corresponding locus of the other homolog, you'll have the wild-type Sb+ allele. Let me draw out the two possibilities:

In both cases, whether the mutant genes are on the same of chromosome or homologous one, regardless, that mutant gene is still present in the cell. The question mentions that the mutations are dominant. Therefore, I was thinking that in either case, since the mutant genes are still present in the cell and they're dominant, both mutant phenotypes will present in either case.

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