Chickens that carry both the alleles for rose comb (R) and pea comb (P) have walnut combs, whereas chickens that lack both of these alleles (i.e., they are genotypically rr pp) have single combs. From the information about interactions between these two genes given in the chapter, determine the phenotypes and proportions expected from the following crosses:

(a) RR Pp × rr Pp;
(b) rr PP × Rr Pp;
(c) Rr Pp × Rr pp;
(d) Rr pp × rr pp.