What does expressivity measure in genetics?

Expressivity refers to the degree to which a genotype manifests itself in the phenotype, indicating that even individuals with the same genetic makeup may display a range of phenotypic traits. It measures the variation in expression of a trait among individuals who share the same genotype, which can occur due to environmental influences, genetic modifiers, or epigenetic factors. For example, two individuals may have the same genetic mutation responsible for a particular condition, but they may show differing symptoms or severity due to expressivity.

This concept is crucial in understanding how genetic conditions can vary widely among individuals, even within the same family, highlighting the complexity of genetic expression. Other choices focus on different aspects of genetics, such as allele frequencies, population genetics, and disease incidence, which do not pertain to the measurement of phenotypic variation stemming from a specific genotype.