Epistasis is the interaction between genes such that one gene influences or interferes with the expression of another gene, leading to a specific phenotype. Epistatic genes can be dominant or recessive. Use your knowledge of epistasis to determine the genotypes of offspring in the following crosses. Part A In freshwater snails, pigment color is influenced by two genes. If two heterozygous pigmented freshwater snails were crossed and offspring were produced in a ratio of 9 pigmented snails to 7 albino snails, what are the genotypes of the offspring?

Pigmented snails: A_B_; albino snails: A_bb, aaB_, aabb

This is an example of recessive epistasis. In recessive epistasis, two recessive alleles mask expression of an allele at a different locus. The 9:7 ratio observed in this question is a special case of epistasis called duplicative recessive epistasis, which means the presence of either aa or bb was sufficient to mask expression of the other gene. In this case, if snails had two copies of either allele a or allele b, pigment production was suppressed, resulting in albino individuals.

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