Final answer:
The proportion of offspring from the cross RrPp x rrPP results in a phenotypic ratio of 1:1 due to the dominance of the R and P alleles.
Step-by-step explanation:
The question asks about calculating the proportion of offspring from a genetic cross between two plants, where one plant has heterozygous alleles RrPp and the other has alleles rrPP. In this scenario, we must first determine the possible gametes each parent can produce and then use a Punnett square to find out the resulting genotypes of the offspring. Since Rr can produce gametes with R or r, and Pp can produce gametes with P or p, the first plant can produce gametes RP, Rp, rP, and rp. The second plant, rrPP, can only produce gametes with rP.
Creating a Punnett square with these gametes would result in offspring with genotypes RrPp, rrPp, RrPP, and rrPP. Given that dominant alleles are capitalized, and in this case R and P are dominant, the phenotypic ratio of the progeny will be 1:1 because any combination with R will show the R phenotype, and all have at least one P, showing the P phenotype.