A 3-inch plant was crossed with a 15-inch plant, and all F₁ plants were 9 inches. The F₂ plants exhibited a 'normal distribution,' with heights of 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 inches.

What will be the outcome if the F₁ plants are testcrossed with plants that are homozygous for all nonadditive alleles?

Verified step by step guidance

1. <span>Identify the genetic basis of the trait: The problem suggests a quantitative trait, as indicated by the normal distribution of plant heights in the F₂ generation. This implies polygenic inheritance, where multiple genes contribute to the phenotype.</span>
2. <span>Understand the F₁ generation: The F₁ plants are all 9 inches tall, which is the average of the parental heights (3 inches and 15 inches). This suggests that the F₁ plants are heterozygous for the alleles contributing to plant height.</span>
3. <span>Define the testcross: A testcross involves crossing the F₁ plants with a plant that is homozygous for all nonadditive alleles. In this context, nonadditive alleles are those that do not contribute to the increase in plant height.</span>
4. <span>Predict the outcome of the testcross: Since the testcross involves crossing heterozygous F₁ plants with homozygous recessive plants, the offspring will exhibit a range of phenotypes. The distribution of these phenotypes will depend on the number of contributing alleles inherited from the F₁ plants.</span>
5. <span>Consider the phenotypic distribution: The offspring of the testcross will likely show a range of heights, but the average height will be lower than that of the F₁ generation, as the homozygous recessive parent contributes no alleles that increase height.</span>

Key Concepts

Quantitative Traits

Quantitative traits are characteristics that are influenced by multiple genes and exhibit a continuous range of phenotypes, such as height. In this case, the plant heights show a normal distribution, indicating that multiple alleles contribute to the trait, resulting in a variety of heights in the F₂ generation.

Testcross

A testcross is a genetic cross between an individual with an unknown genotype and a homozygous recessive individual. This method helps determine the genotype of the unknown individual by analyzing the phenotypes of the offspring. In this scenario, crossing the F₁ plants with homozygous nonadditive alleles will reveal the genetic contributions of the F₁ plants.

Additive vs. Nonadditive Alleles

Additive alleles contribute to the phenotype in a cumulative manner, while nonadditive alleles do not follow this pattern and can mask the effects of other alleles. In the context of the question, crossing F₁ plants with homozygous nonadditive alleles will help clarify how these alleles influence the resulting plant heights and the overall distribution of traits in the offspring.