Expressivity is a term used in genetics that refers to variations of a phenotype in individuals carrying a particular genotype. The term can be used to qualitatively or quantitatively characterize the extent of the phenotype variation given a particular genotype. The term is equivalent to the severity of a condition in clinical medicine. For example, the amount of blood ejected from the pumping heart with each contraction can be quantitated 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. This differs from penetrance in that penetrance refers to the likelihood of the gene generating any phenotype at all, while expressivity refers to the influence of an expressed gene in individuals. 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 gene involved in a quantitative trait like body height might have large variance (some are taller 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.
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