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

Verified step by step guidance

1. <span>Step 1: Identify the genotypes that correspond to each phenotype. In a dihybrid cross with genes R and T, the F₂ progeny can have the following genotypes: R_T_, R_tt, rrT_, and rrtt. Each of these genotypes will correspond to a different phenotype based on the interaction of the genes.</span>
2. <span>Step 2: Assign phenotypes to each genotype based on the given ratio. The 9/16 black phenotype is typically associated with the presence of both dominant alleles (R_T_), the 3/16 gray phenotype with one dominant allele (R_tt or rrT_), and the 4/16 albino phenotype with the absence of both dominant alleles (rrtt).</span>
3. <span>Step 3: Propose a biochemical pathway. Assume that the colorless precursor is converted to a black pigment by the action of both dominant alleles (R and T). If only one dominant allele is present, the precursor is partially converted to a gray pigment. If neither dominant allele is present, the precursor remains unmodified, resulting in an albino phenotype.</span>
4. <span>Step 4: Describe the role of each gene. Gene R catalyzes the first step in the pathway, converting the precursor to an intermediate. Gene T catalyzes the second step, converting the intermediate to the final black pigment. If either step is incomplete due to a recessive allele, the pathway results in a gray pigment or remains unmodified.</span>
5. <span>Step 5: Explain the genetic interaction. This is an example of complementary gene action, where both genes are required for the full expression of the black phenotype. The presence of at least one dominant allele at each locus is necessary for the complete conversion of the precursor to the black pigment.</span>

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.