Final answer:
The modified phenotypic ratio produced by a dominant epistasis relationship between two genes can significantly deviate from the Mendelian 9:3:3:1 ratio. In the case of the shepherd's purse plant, this results in a 15:1 ratio of triangular to ovoid seeds when considering a cross between heterozygotes for the interacting genes. Dominant epistasis changes expected phenotypic ratios by allowing one gene's dominance to mask the expression of another.
Step-by-step explanation:
The modified ratio produced by a dominant epistasis relationship between two genes is typically not the traditional 9:3:3:1 ratio but rather a different ratio that reflects the epistatic interaction. Consider the example of the shepherd's purse plant (Capsella bursa-pastoris), where seed shape is controlled by two genes in a dominant epistatic relationship. In this case, the dominant alleles of either of these genes, A or B, will lead to the same triangular seed phenotype. A cross between heterozygotes for both genes (AaBb x AaBb) will give a phenotypic ratio of 15 triangular seeds to 1 ovoid seed because any genotype other than homozygous recessive for both genes (aabb) will result in triangular seeds.
When working through genetics problems with two-gene interactions, it's important to remember that epistasis can alter the expected ratios that are based on independent assortment and simple dominance. This scenario, where one gene masks the expression of another gene, affects the expected phenotypic ratio. Rather than seeing two separate 3:1 ratios as you would in non-interacting gene pairs, dominant epistasis creates a scenario where one gene's dominance overshadows the effects of the other, thus changing the overall phenotypic expression and creating unique modified phenotypic ratios.