Question

While most quantitative traits display continuous variation, there are others referred to as “threshold traits” that are distinguished by having a small number of discrete phenotypic classes. For example, Type 2 diabetes (adult-onset diabetes) is considered to be a polygenic trait, but demonstrates only two phenotypic classes: individuals who develop the disease and those who do not. Theorize how a threshold trait such as Type 2 diabetes may be under the control of many polygenes, but express a limited number of phenotypes.

Accepted Answer

Understand that threshold traits, like Type 2 diabetes, are influenced by multiple genes (polygenes) that each contribute a small effect to an underlying continuous variable called liability or risk. Recognize that although the genetic and environmental factors create a continuous distribution of liability in the population, the phenotype is expressed in discrete classes based on whether an individual's liability exceeds a certain threshold. Visualize the concept by imagining a bell-shaped curve representing the distribution of liability; individuals whose combined genetic and environmental risk surpasses the threshold develop the disease, while those below do not. Consider that the threshold acts as a cutoff point, converting the continuous variation in liability into a binary phenotype (disease or no disease), which explains why only a few phenotypic classes are observed despite polygenic control. Summarize that the polygenic inheritance creates a continuous risk spectrum, but the phenotype appears discrete because expression depends on crossing a liability threshold influenced by both genes and environment.

Key Concepts

Polygenic Inheritance

Threshold Traits

Liability-Threshold Model