Expressivity

Expressivity is a term used in genetics to refer to variations in a phenotype among individuals carrying a particular genotype. The term can be used to characterize qualitatively or quantitatively the extent of phenotypic variation given a particular genotype. The term is analogous to the severity of a condition in clinical medicine. For example, the volume of blood ejected from the pumping heart with each contraction, relative to the total amount of blood contained in the heart's chamber can be quantified by echocardiography and is called the ejection fraction. If a specific genotype is associated with the development of congestive heart failure, the expressivity would be represented by the range of ejection fractions seen in patients that have that genotype. As a more qualitative example, the "blue" gene might have an expressivity of 25% for individuals that express the "blue" gene and appear light blue, and 75% for individuals that express the "blue" gene and appear dark blue. Expressivity is measured only when there is 100% penetrance.

This differs from penetrance, which refers to the likelihood of the gene generating its associated phenotype at all, as determined by the proportion of individuals with a given genotype who also possess the associated phenotype. In contrast, expressivity refers to the influence of an expressed gene at the level of particular individuals.