How do we know that threshold traits are actually polygenic even though they may have as few as two discrete phenotypic classes?

Verified step by step guidance

1. Understand that threshold traits are those that exhibit a binary phenotype, such as 'affected' or 'unaffected', but are influenced by multiple genes.
2. Recognize that these traits are polygenic, meaning they are controlled by several genes, each contributing a small effect to the overall phenotype.
3. Consider the concept of liability, which is an underlying continuous variable influenced by genetic and environmental factors. An individual expresses the trait only if their liability exceeds a certain threshold.
4. Examine family studies and population data that show a continuous distribution of liability, even though the phenotype is binary, supporting the polygenic nature of these traits.
5. Look at examples such as type 2 diabetes or schizophrenia, where genetic studies have identified multiple loci contributing to the risk, indicating a polygenic basis despite the binary outcome.

Key Concepts

Threshold Traits

Threshold traits are characteristics that exhibit a discrete phenotype, but their expression is influenced by multiple genetic factors. These traits typically show a clear cutoff point, where individuals either express the trait or do not, despite the underlying continuous variation in the contributing genetic factors.

Polygenic Inheritance

Polygenic inheritance refers to the phenomenon where multiple genes contribute to a single trait, resulting in a continuous range of phenotypes. Even when a trait appears to have only a few discrete classes, the cumulative effect of many alleles can lead to a threshold effect, where a certain genetic 'threshold' must be crossed for the trait to manifest.

Phenotypic Classes

Phenotypic classes are distinct categories of observable traits that result from the interaction of genetic and environmental factors. In the context of threshold traits, even with limited phenotypic classes, the underlying genetic architecture can be complex, indicating that multiple genes are involved in determining the trait's expression.