Question

While most quantitative traits display continuous variation, there are others referred to as "threshold traits" that are distin- guished 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.

Short answer

Answer: Type 2 diabetes, a threshold trait, is influenced by multiple genes and various environmental factors. Although many genes contribute to the overall genetic risk or liability for developing Type 2 diabetes, the accumulation of these genetic effects and environmental factors determines whether an individual crosses the threshold needed to develop the disease. As a result, only a limited number of phenotypes are observed – people who develop Type 2 diabetes and those who do not.

Step-by-step solution

Understanding Threshold Traits

Threshold traits are considered as quantitative traits that are controlled by multiple genes (polygenic) but exhibit only a limited number of distinct phenotypic categories. These traits follow a continuous distribution in the population, but only individuals who cross a particular threshold will exhibit the particular phenotype.

Explaining Threshold Traits Mechanism

An individual's phenotype for a threshold trait is determined by a combination of the sum of the effects of multiple genes, as well as environmental factors and epistatic interactions that influence the trait. Each individual accumulates a certain amount of liability for a specific trait, in this case, Type 2 diabetes. If the individual's combined liability crosses the threshold, they will develop the disease, and if not, they will not develop the disease. Hence, we see the limited number of phenotypes (disease or no disease) even though many genes are involved.

Genetic Factors in Type 2 Diabetes as a Threshold Trait

Type 2 diabetes is a complex disease influenced by several genetic and environmental factors. As a polygenic trait, there are multiple genes that contribute to the development of Type 2 diabetes by affecting insulin sensitivity, insulin production, and glucose metabolism. Each gene independently contributes to the overall genetic risk or liability for developing Type 2 diabetes. This accumulation of genetic effects can push an individual past the threshold needed to develop the disease.

Environmental Factors in Type 2 Diabetes as a Threshold Trait

Apart from genetic factors, environmental factors play a significant role in determining an individual's predisposition to develop Type 2 diabetes. Factors like diet, fitness level, and stress influence the tendency for an individual to become insulin resistant, or be unable to produce sufficient insulin. These environmental factors add to or subtract from the accumulation of liability, and collectively determine if an individual will cross the threshold to develop the disease.

The Limited Phenotypes of Type 2 Diabetes as Threshold Trait

Since Type 2 diabetes is considered a threshold trait, only a limited number of phenotypic categories are observed in the population: people who develop Type 2 diabetes and those who do not. The underlying genetic and environmental factors contribute to the liability of an individual towards developing Type 2 diabetes; however, it's the accumulation of these factors that push them past the threshold, resulting in either the presence or absence of the disease.