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 red : 7/16 white

Verified step by step guidance

1. <span>Step 1: Identify the genotypes that contribute to the phenotypic ratio. In a dihybrid cross of RrTt x RrTt, the F₂ generation will have a 9:3:3:1 genotypic ratio. However, we need to explain a 9:16 red : 7:16 white phenotypic ratio.</span>
2. <span>Step 2: Determine the role of each gene. Assume that the dominant allele R catalyzes the first step in the pathway, converting a colorless precursor to an intermediate. The dominant allele T catalyzes the second step, converting the intermediate to a red pigment.</span>
3. <span>Step 3: Define the phenotypes based on genotypes. The red phenotype (9/16) occurs when both R and T are present and functional (R_T_). The white phenotype (7/16) occurs when either or both steps are blocked, i.e., when the genotype is rrT_, R_tt, or rrtt.</span>
4. <span>Step 4: Explain the genetic interaction. This is an example of complementary gene action, where both genes are required for the final product (red pigment) to be produced. If either gene is homozygous recessive, the pathway is blocked, resulting in a white phenotype.</span>
5. <span>Step 5: Summarize the pathway. The pathway starts with a colorless precursor. The presence of at least one dominant R allele allows conversion to an intermediate. The presence of at least one dominant T allele allows conversion of the intermediate to a red pigment. If either step is blocked by recessive alleles, the result is a white phenotype.</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 scenario, the genes R and T are being studied, where each gene can have a dominant or recessive allele. The F₂ progeny ratios arise from the combination of these alleles during gamete formation and fertilization, illustrating Mendelian inheritance patterns.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In this case, the 9:7 ratio indicates that certain combinations of dominant alleles lead to red pigment production, while others result in a white or albino phenotype. Understanding how these ratios arise from gene interactions is crucial for explaining the observed outcomes in the F₂ generation.

Gene Interaction

Gene interaction occurs when the effects of one gene are modified by one or more other genes, leading to a combined effect on the phenotype. In this question, the dominant alleles of genes R and T catalyze different reactions that contribute to pigment production. The recessive alleles are loss-of-function, meaning they do not produce a functional product, which can significantly alter the resulting phenotypic ratios in the progeny.