Final answer:
Epistasis is a form of genetic interaction where one gene masks the expression of another gene, which differs from simple dominance which occurs when a dominant allele masks the expression of a recessive allele within the same gene. In the context of the shepherd's purse plant, epistasis results in triangular seeds unless both genes are homozygous recessive, leading to a 15:1 phenotypic ratio when two heterozygotes are crossed.
Step-by-step explanation:
Epistasis is a genetic phenomenon where one gene interferes with or masks the expression of another gene. The term epistasis comes from Greek roots meaning "standing upon." An epistatic gene that does the masking is referred to as epistatic, while the gene whose expression is masked is called hypostatic. This relationship can profoundly affect the phenotypic ratios seen in genetic crosses, different from the simple dominant-recessive relationships seen in Mendelian inheritance. The biochemical basis for epistasis often involves gene pathways where one gene's function is necessary for another gene further down the pathway to be expressed.
Dominance vs. Epistasis
While dominance refers to the interaction between different alleles of the same gene, with the dominant allele masking the expression of a recessive allele, epistasis involves interactions between different genes. Epistasis can be complex, with multiple gene interactions leading to variations in phenotype that are not predictable by simple dominance. For instance, in shepherd's purse plants (Capsella bursa-pastoris), the presence of a dominant allele in any of two genes results in triangular seeds, overriding the seed shape that would be produced by the other gene. A phenotype of ovoid seeds only occurs when there are two homozygous recessive genes (aabb). Hence, crossing two heterozygotes for each gene (AaBb x AaBb) yields a phenotypic ratio of 15 triangular to 1 ovoid.