Two genes interact to produce various phenotypic ratios among F₂ progeny of a dihybrid cross. Design a different pathway explaining each of the F₂ ratios below, using hypothetical genes R and T and assuming that the dominant allele at each locus catalyzes a different reaction or performs an action leading to pigment production. The recessive allele at each locus is null (loss-of-function). Begin each pathway with a colorless precursor that produces a white or albino phenotype if it is unmodified. The ratios are for F₂ progeny produced by crossing wild-type F₁ organisms with the genotype RrTt.

9/16 black : 3/16 gray : 4/16 albino

Key Concepts

Dihybrid Cross

A dihybrid cross involves two traits, each controlled by different genes, typically represented by two pairs of alleles. In this case, the genes R and T are being analyzed for their interactions in producing phenotypes. The classic Mendelian ratio for a dihybrid cross of two heterozygous parents (RrTt x RrTt) is 9:3:3:1, but in this scenario, the interaction between the genes modifies the expected ratios, leading to a more complex outcome.

Epistasis

Epistasis occurs when the expression of one gene is affected by another gene, leading to modified phenotypic ratios. In this question, the dominant alleles of genes R and T interact in such a way that they influence pigment production pathways. Understanding how these genes interact is crucial for explaining the observed ratios of black, gray, and albino phenotypes among the F₂ progeny.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this case, the ratios of 9/16 black, 3/16 gray, and 4/16 albino indicate how the interactions between the alleles of genes R and T lead to varying levels of pigment production. Analyzing these ratios helps in constructing the pathways that explain the genetic interactions at play.