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.
15/16 black : 1/16 white

Verified step by step guidance

1. Step 1: Understand the problem. The F₂ phenotypic ratio of 15/16 black : 1/16 white suggests that two genes (R and T) interact in a way where the presence of at least one dominant allele at either locus (R or T) is sufficient to produce the black phenotype. The white phenotype occurs only when both loci are homozygous recessive (rrtt).
2. Step 2: Define the pathway. Start with a colorless precursor. The dominant allele at each locus (R or T) catalyzes a reaction leading to the production of black pigment. If both loci are homozygous recessive (rrtt), the precursor remains unmodified, resulting in a white phenotype.
3. Step 3: Assign roles to the genes. Hypothesize that gene R encodes an enzyme that converts the colorless precursor into an intermediate product, and gene T encodes an enzyme that converts the intermediate product into the final black pigment. Either enzyme alone can complete the pathway to produce black pigment.
4. Step 4: Explain the genetic interaction. In the F₂ generation, the genotypes R-T-, R-tt, and rrT- all result in black pigment because at least one dominant allele (R or T) is present to catalyze the reaction. Only the rrtt genotype lacks both functional enzymes, leaving the precursor unmodified and resulting in a white phenotype.
5. Step 5: Verify the ratio. Calculate the expected phenotypic ratio based on the dihybrid cross RrTt x RrTt. Use a Punnett square to determine the genotypic combinations. The phenotypic ratio of 15/16 black (R-T-, R-tt, rrT-) to 1/16 white (rrtt) matches the observed data, confirming the pathway's validity.

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 phenotypic ratios observed in the offspring result from the independent assortment of these alleles during gamete formation, leading to a variety of combinations in the F₂ generation.

Dominant and Recessive Alleles

In genetics, dominant alleles are those that express their trait even when only one copy is present, while recessive alleles require two copies to manifest their trait. In this case, the dominant alleles R and T lead to pigment production, while their recessive counterparts are null alleles that do not produce any functional product. Understanding this distinction is crucial for predicting the phenotypic outcomes of the cross.

Phenotypic Ratios

Phenotypic ratios represent the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In the given scenario, the ratio of 15/16 black to 1/16 white indicates that most offspring exhibit the dominant phenotype due to the presence of at least one dominant allele from either gene. This ratio can be explained through the interaction of the genes R and T in a biochemical pathway that leads to pigment production.