Final answer:
Epistasis is when one gene affects the expression of another, leading to phenotypic ratios that can diverge from the expected 9:3:3:1 in a dihybrid cross.
Step-by-step explanation:
Understanding Epistasis in F2 Generation
In inheritance, epistasis is a phenomenon where one gene can interfere with the expression of another. Considering a dihybrid cross that involves two independently assorting genes that determine phenotype, we can expect a modified phenotypic ratio in the F2 generation due to this interaction. Specifically, in the question's case, the genes for brown and scarlet are both recessive and when they interact, they lead to a white phenotype.
In the typical Mendelian dihybrid cross, the expectation is a 9:3:3:1 ratio in the F2 generation. However, due to epistasis, the phenotype ratios can significantly deviate from this. For example, with the gene interaction in mice pigmentation, a cross between AaCc x AaCc heterozygotes for two genes results in a phenotypic ratio of 9 agouti:3 solid color:4 albino. This is because the C gene is epistatic to the A gene, meaning presence of cc will result in an albino mouse, regardless of the A gene.
For the student's specific case involving independently assorting genes bw (brown) and st (scarlet), we expect the genotypes with both recessive alleles (bw/bw; st/st) to produce a white phenotype due to the interaction of these genes. Other genotypes would produce their respective colors unless this specific recessive epistasis interaction occurs.