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 green : 3/16 yellow : 3/16 blue : 1/16 white

Verified step by step guidance

1. span>Step 1: Identify the phenotypic classes and their ratios. The F₂ progeny show a 9:3:3:1 ratio, which is typical of a dihybrid cross involving two independently assorting genes.</span
2. span>Step 2: Assign hypothetical functions to the genes R and T. Assume that the dominant allele R catalyzes a reaction converting a colorless precursor to a yellow pigment, and the dominant allele T catalyzes a reaction converting the yellow pigment to a green pigment.</span
3. span>Step 3: Determine the genotypes responsible for each phenotype. The green phenotype (9/16) results from the presence of both dominant alleles (R_T_), the yellow phenotype (3/16) results from the presence of the dominant R allele and the recessive t allele (R_tt), the blue phenotype (3/16) results from the presence of the recessive r allele and the dominant T allele (rrT_), and the white phenotype (1/16) results from the presence of both recessive alleles (rrtt).</span
4. span>Step 4: Construct a pathway diagram. Start with a colorless precursor. The presence of R converts it to yellow, and the presence of T converts yellow to green. If R is absent, the precursor remains colorless unless T is present, which converts it to blue. If both R and T are absent, the precursor remains colorless (white).</span
5. span>Step 5: Verify the pathway with the given phenotypic ratios. Check that the genotypic combinations match the expected phenotypic ratios: 9/16 green (R_T_), 3/16 yellow (R_tt), 3/16 blue (rrT_), and 1/16 white (rrtt).</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 various phenotypes. The classic Mendelian ratio for a dihybrid cross of two heterozygous parents (RrTt x RrTt) is 9:3:3:1, but here we are examining a modified ratio due to gene interactions affecting pigment production.

Gene Interaction

Gene interaction occurs when two or more genes influence the outcome of a single trait, leading to phenotypic ratios that differ from those expected from independent assortment. In this scenario, the dominant alleles of genes R and T catalyze different reactions that modify a colorless precursor into various pigments, resulting in the observed phenotypic ratios of green, yellow, blue, and white in the F₂ progeny.

Loss-of-Function Alleles

Loss-of-function alleles are mutations that result in the complete or partial inactivation of a gene, leading to a lack of functional protein. In this context, the recessive alleles (r and t) are null alleles that do not produce any functional product, resulting in a white or albino phenotype when both alleles are recessive. Understanding how these alleles interact with their dominant counterparts is crucial for explaining the phenotypic ratios in the progeny.