A three-gene system of additive genes (A, B, and C) controls plant height. Each gene has two alleles (A and a, B and b, and C and c). There is dominance among the alleles of each gene, with alleles A, B, and C dominant over a, b, and c. Under this scheme, the dominant genotype for a gene contributes 10 cm to height potential, and the recessive genotype contributes 4 cm. What are the phenotypes and proportions of each phenotype among the F₂?

Key Concepts

Additive Gene Action

Additive gene action refers to the cumulative effect of multiple genes on a single trait, where each dominant allele contributes a specific amount to the phenotype. In this case, the three genes (A, B, and C) each add to the plant height, with dominant alleles contributing more than recessive ones. Understanding this concept is crucial for predicting the overall phenotype based on the combination of alleles present.

Genotype and Phenotype

The genotype is the genetic makeup of an organism, while the phenotype is the observable expression of that genotype. In this scenario, the different combinations of alleles (AA, Aa, aa, etc.) will determine the plant's height, which is the phenotype. Recognizing how genotypes translate into phenotypes is essential for determining the proportions of each phenotype in the offspring.

Punnett Square and F₂ Generation

A Punnett square is a tool used to predict the genetic outcomes of a cross between two organisms. In the F₂ generation, the offspring are produced from the self-fertilization of F₁ hybrids, allowing for a variety of genotype combinations. Analyzing the Punnett square for the three-gene system will help determine the expected phenotypic ratios among the F₂ plants based on the additive effects of the alleles.