Three pairs of genes with two alleles each (A₁ and A₂, B₁ and B₂, and C₁ and C₂) control the height of a plant. The alleles of these genes have an additive relationship: Each copy of alleles A₁, B₁, and C₁ contributes 6 cm to plant height, and each copy of alleles A₂, B₂, and C₂ contributes 3 cm.

What are the expected heights of plants with each of the homozygous genotypes A₁A₁B₁B₁C₁C₁ and A₂A₂B₂B₂C₂C₂. 

Key Concepts

Additive Gene Interaction

Additive gene interaction occurs when multiple genes contribute to a single trait, with each allele having a specific effect. In this case, the alleles A₁, B₁, and C₁ each add 6 cm to the plant's height, while A₂, B₂, and C₂ add 3 cm. This means that the total height of the plant is the sum of the contributions from all alleles present.

Homozygous Genotypes

A homozygous genotype consists of two identical alleles for a particular gene. In the question, A₁A₁, B₁B₁, and C₁C₁ represent homozygous dominant genotypes, while A₂A₂, B₂B₂, and C₂C₂ represent homozygous recessive genotypes. Understanding these genotypes is crucial for calculating the expected plant heights based on the additive contributions of the alleles.

Phenotypic Expression

Phenotypic expression refers to the observable traits of an organism, which result from the interaction of its genotype with the environment. In this scenario, the height of the plants is the phenotype influenced by the specific combinations of alleles. By calculating the contributions from the homozygous genotypes, we can predict the expected phenotypic heights of the plants.