In chickens, comb shape is determined by genes at two loci (R, r and P, p).A walnut comb is produced when at least one dominant gene R is present at one locus and at least one dominant gene P is present at a second locus (genotype R_ P_).A rose comb is produced when at least one dominant gene is present at the first locus and two recessive genes are present at the second locus (genotype R_ pp).A pea comb is produced when two recessive genes are present at the first locus and at least one dominant gene is present at the second (genotype rr P_).If two recessive genes are present at the first and the second locus (rr pp), a single comb is produced.Give genotypes for comb shape of the parents in the following cross: Walnut crossed with single produces one walnut, one rose, one pea, and one single offspring.

Question

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- The cross between a walnut comb (R_ P_) and a single comb (rr pp) can only produce offspring with genotypes Rr Pp, Rr pp, rr Pp, and rr pp.
- The offspring in the question are one walnut (R_ P_), one rose (R_ pp), one pea (rr P_), and one single (rr pp), which means the parents must both carry at least one dominant gene for each locus. 
- This rules out choice A (RR PP × rr pp), which has one parent that is homozygous recessive for both loci.
- It also rules out choice B (RR Pp × rr pp), which has one parent that is heterozygous for both loci, but the other parent is homozygous recessive for both loci.
- Choice C (Rr PP × rr pp) can only produce offspring with genotypes Rr Pp and rr Pp, so it cannot explain the rose and single offspring.
- Choice E (Rr pp × rr pp) can only produce offspring with genotypes Rr pp and rr pp, so it cannot explain the walnut and pea offspring.
- This leaves choice D (Rr Pp × rr pp) as the only option that can explain all of the observed offspring genotypes.

In Chickens, Comb Shape Is Determined by Genes at Two