In genetics, expressivity quantifies variation in a non-binary phenotype across individuals carrying a particular genotype. It is equal to the proportion of individual carriers of a genotype for a trait who show the trait to a specifiable extent (as in the "shades of blue" example below). This differs from penetrance, the term that measures how often a gene generates its associated phenotype to any extent that makes an individual carrier different from the wild type. Expressivity therefore characterizes non-binary qualitatively or quantitatively the extent of phenotypic variation within a particular genotype. With binary phenotypes (e.g., albino vs. wild type) expressivity and penetrance are the same. 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, in the hypothetical example of a "blue" gene, the gene might have an expressivity of 25% for individuals that express the "blue" gene and appear light blue, while for individuals who express the "blue" gene and appear dark blue, the expressivity might be 75%.
Variable expressivity occurs when a phenotype is expressed to a different degree among individuals with the same genotype. For example, individuals with the same allele for a gene involved in a quantitative trait like body height might have large variance (some are heavier than others), making prediction of the phenotype from a particular genotype alone difficult. The expression of a phenotype may be modified by the effects of aging, other genetic loci, or environmental factors.