Height in humans depends on the additive action of genes. Assume that this trait is controlled by the four loci R, S, T, and U and that environmental effects are negligible. Instead of additive versus nonadditive alleles, assume that additive and partially additive alleles exist. Additive alleles contribute two units, and partially additive alleles contribute one unit to height.

Can two individuals of moderate height produce offspring that are much taller or shorter than either parent? If so, how?

Verified step by step guidance

1. <span>Step 1: Understand the genetic model: The problem states that height is controlled by four loci (R, S, T, U) with additive and partially additive alleles. Additive alleles contribute 2 units to height, while partially additive alleles contribute 1 unit.</span>
2. <span>Step 2: Define moderate height: Determine what constitutes moderate height in terms of the number of additive and partially additive alleles. For example, if each locus can have two alleles, a moderate height individual might have a mix of additive and partially additive alleles.</span>
3. <span>Step 3: Calculate potential allele combinations: Consider the possible combinations of alleles from the parents. Each parent can pass on different combinations of alleles to their offspring, leading to a range of possible genotypes in the offspring.</span>
4. <span>Step 4: Determine the range of offspring heights: Calculate the minimum and maximum possible height contributions from the alleles that the offspring could inherit. This involves considering the extremes of inheriting all additive alleles or all partially additive alleles.</span>
5. <span>Step 5: Analyze the potential for variation: Compare the range of possible offspring heights to the parents' heights. If the range includes values much higher or lower than the parents' heights, then it is possible for the offspring to be much taller or shorter than the parents.</span>

Key Concepts

Additive and Partially Additive Alleles

Additive alleles contribute a fixed amount to a trait, in this case, height, with each allele adding two units. Partially additive alleles contribute less, specifically one unit. The combination of these alleles from both parents can lead to a range of phenotypic expressions in offspring, allowing for variability in traits like height.

Polygenic Inheritance

Height is a polygenic trait, meaning it is influenced by multiple genes (loci). In this scenario, four loci (R, S, T, U) are involved, and the cumulative effect of the alleles at these loci determines the overall height of an individual. This complexity allows for a wide range of possible heights in offspring, even if both parents are of moderate height.

Phenotypic Variation

Phenotypic variation refers to the observable differences in traits among individuals, which can arise from genetic differences and the interaction of multiple alleles. In the context of height, two moderate-height parents can produce offspring that are significantly taller or shorter due to the various combinations of additive and partially additive alleles inherited from both parents, leading to a diverse range of heights.