Expressivity

Expressivity is a term used in genetics that refers to variations of a phenotype in individuals carrying a particular genotype. The term is used to qualitatively characterize the variance or extent of the phenotype given a particular genotype. For example, the "blue" gene might have an expressivity of 25 or individuals that express the "blue" gene and appear light blue, and 75 or 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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The consistency of a genetic disease. For example, Marfan disease shows variable expressivity. Some persons with Marfan's merely have long fingers and toes while others have the full-blown disease with dislocation of the lens and dissecting aneurysm of the aorta.

The range of phenotypes resulting from a given genotype (cystic fibrosis, for example, may have a variable degree of severity). This is different from pleiotropy which refers to a variety of different phenotypes resulting from the same genotype.

expressivity
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