What is the expected phenotypic ratio of a dihybrid cross of two heterozygous parents?

The expected phenotypic ratio of a dihybrid cross between two heterozygous parents (for example, both parents being RrYy, where R and r are alleles for one trait and Y and y are alleles for another trait) is indeed 9:3:3:1.

This ratio arises because each trait segregates independently due to Mendel's law of independent assortment. In a dihybrid cross, there are four possible allele combinations for each parent (RY, Ry, rY, and ry), leading to 16 potential combinations for the offspring. When you categorize these combinations based on the phenotypes of the two traits, you find that the distribution of phenotypes results in nine offspring exhibiting the dominant traits for both characters, three exhibiting the dominant phenotype for the first trait and the recessive for the second, three exhibiting the recessive phenotype for the first trait and the dominant for the second, and one exhibiting the recessive phenotype for both traits.

This 9:3:3:1 ratio reflects the probability of different phenotypes showing up in the offspring when two genes that assort independently are taken into account.