Question

In fruit flies, the gene for red eyes (R) is dominant and the gene for sepia eyes (r) is recessive. If two red-eyed heterozygous flies (Rr) mate together, what are the possible genotypes of their offspring? (Hint: You many want to complete a Punnett square.)

Answer 1

You have 2 parents both Rr, so the cross will simply be Rr x Rr. Drawing a punnet square, your cross should look like this:$\left[\begin{array}{ccc}-&R&r\\R&RR&Rr\\r&Rr&rr\end{array}\right]$
We see that when 2 heterozygotes cross, the possible genotypes for their offspring are RR, Rr, and rr. The punnet square tells us that 25% of the offspring will be RR, 50% will be Rr, and 25% will be rr. Phenotypically, this means 75% of offspring will have red eyes, and 25% will have sepia.