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

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.