Final answer:
Epistasis affects modified dihybrid phenotypic ratios by causing deviations from expected Mendelian ratios when one gene interacts with and masks the expression of another. An example is the shepherd's purse plant, where the presence of a dominant allele in either of two genes results in a phenotypic ratio of 15 triangular seeds to 1 ovoid seed, instead of the 9:3:3:1 ratio expected in a typical dihybrid cross.
Step-by-step explanation:
Epistasis often affects modified dihybrid phenotypic ratios by altering the expected Mendelian ratios when genes interact with each other. For example, if one gene masks the expression of another gene, as in the case where coat color is determined by two genes where the presence of a certain allele in one gene (C) masks the expression of the other gene (A), phenotypic ratios can differ significantly from the expected 9:3:3:1 seen in dihybrid crosses involving independent assortment and non-interacting genes.
In the case of the shepherd's purse plant (Capsella bursa-pastoris), a dominant epistatic relationship exists between two genes that determine seed shape. With dominant epistasis, where either A or B being dominant (having at least one dominant allele) results in a triangular seed phenotype, the expected phenotypic ratio from a heterozygote cross (AaBb x AaBb) is significantly altered to 15 triangular : 1 ovoid, rather than the standard 9:3:3:1 ratio. This is because all genotypes except for homozygous recessive for both genes (aabb) result in the same phenotype.
Understanding that interacting gene pairs can still assortment independently but will have altered phenotypic ratios due to the epistatic interactions is essential when predicting outcomes in genetics problems. This interaction exemplifies how epistasis can lead to modified dihybrid phenotypic ratios that deviate from Mendel's classic dihybrid ratios.