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. Understand the genetic model: Height is controlled by four loci (R, S, T, U), each with alleles that contribute additively to height. Additive alleles contribute 2 units, and partially additive alleles contribute 1 unit to height. Environmental effects are negligible, so phenotype depends solely on genotype.
2. Define the genotype of the parents: Since both parents are of moderate height, they likely have a mix of additive and partially additive alleles across the four loci. For example, each parent might have some loci with additive alleles and others with partially additive alleles, resulting in an intermediate total height contribution.
3. Consider the possible gametes and offspring genotypes: Each parent can pass on different combinations of alleles at the four loci. Because alleles contribute additively, offspring genotypes can vary widely depending on which alleles they inherit from each parent.
4. Calculate the range of possible offspring heights: By summing the contributions of alleles inherited at each locus (2 units for additive, 1 unit for partially additive), offspring can have total height contributions that are either higher or lower than either parent, depending on allele combinations.
5. Conclude that offspring can be much taller or shorter than parents: Due to the combination of additive and partially additive alleles segregating independently, offspring can inherit more additive alleles than either parent (resulting in taller height) or fewer (resulting in shorter height), explaining how moderate-height parents can produce offspring with more extreme heights.

Key Concepts

Polygenic Inheritance

Polygenic inheritance refers to a trait controlled by multiple genes, each contributing a small effect to the phenotype. In this case, height is influenced by four loci (R, S, T, U), where the combined effect of alleles determines the overall height. This leads to continuous variation in traits like height rather than discrete categories.

Additive and Partially Additive Alleles

Additive alleles contribute a fixed amount to the trait value, while partially additive alleles contribute a smaller, intermediate amount. Here, additive alleles add two units to height, and partially additive alleles add one unit. The total phenotype is the sum of these contributions across all loci.

Genetic Variation and Recombination in Offspring

Offspring inherit different combinations of alleles from their parents due to independent assortment and recombination. Even if parents have moderate height, their offspring can inherit more additive alleles or fewer partially additive alleles, resulting in taller or shorter phenotypes than either parent.