In chickens, comb shape is determined by two independent genes (R and P). A walnut comb is produced when at least one dominant allele R is present at one gene locus and at least one dominant allele P is present at a second gene locus (genotype R_ P_). A rose comb is produced when at least one dominant allele is present at the first locus and two recessive alleles are present at the second locus (genotype R_ pp). A pea comb is produced when two recessive alleles are present at the first locus and at least one dominant allele is present at the second (genotype rr P_). If two recessive alleles 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: Rose crossed with pea produces 2 walnut, 1 single, and 1 pea offspring.
a) RR pp × rr PP
b) Rr pp × rr Pp
c) Rr pp × rr PP
d) RR pp × rr Pp
e) Rr pp × Rr Pp