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 dark blue : 6/16 light blue : 1/16 white

Verified step by step guidance

1. span>Step 1: Identify the genotypes that correspond to each phenotype. In a dihybrid cross, the F₂ generation results from crossing two heterozygous individuals (RrTt). The possible genotypes are RRTT, RRTt, RrTT, RrTt, RRTt, RrTt, Rrtt, rrTT, rrTt, and rrtt.</span
2. span>Step 2: Assign phenotypes to each genotype based on the given ratios. The 9/16 dark blue phenotype suggests that both dominant alleles (R and T) are required for the dark blue color. Therefore, genotypes RRTT, RRTt, RrTT, and RrTt produce dark blue.</span
3. span>Step 3: Determine the genotypes for the 6/16 light blue phenotype. This phenotype likely results from having one dominant allele at either locus, but not both. Thus, genotypes Rrtt and rrTt produce light blue.</span
4. span>Step 4: Assign the white phenotype to the remaining genotype. The 1/16 white phenotype occurs when both loci have recessive alleles, resulting in the genotype rrtt.</span
5. span>Step 5: Describe the biochemical pathway. Assume that the R allele catalyzes the first step in the pathway, converting a colorless precursor to a light blue intermediate. The T allele catalyzes the second step, converting the light blue intermediate to a dark blue pigment. If either allele is recessive, the pathway is incomplete, resulting in a light blue or 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 case, 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 (F₂ generation) arise from the independent assortment of these alleles during gamete formation, leading to a variety of combinations in the progeny.

Phenotypic Ratios

Phenotypic ratios describe the relative frequencies of different phenotypes in the offspring resulting from a genetic cross. In the given scenario, the ratios of 9/16 dark blue, 6/16 light blue, and 1/16 white indicate how the interactions between the alleles of genes R and T influence pigment production. Understanding these ratios is crucial for predicting the outcomes of genetic crosses and the underlying genetic mechanisms.

Gene Interaction

Gene interaction occurs when the effects of one gene are modified by one or several other genes, leading to a combined effect on the phenotype. In this case, the dominant alleles of genes R and T catalyze different reactions that contribute to pigment production from a colorless precursor. The specific interactions between these genes can result in various phenotypes, as seen in the distinct ratios of the F₂ progeny.