expressivity

The degree of manifestation of a gene in those individuals in whom its presence is detectable. The gene for tuberose sclerosis, for example, in one individual may cause epilepsy, mental retardation, and a skin rash, but in another only the skin condition. Such cases are said to be of variable expressivity.

Expressivity refers to variations of a phenotype in genetics. The term is used to qualitatively characterize the variance or extent of the phenotype. For example a quantitative trait - like body height - might have large variance and therefore can make prediction of the phenotype difficult. In these instances expressivity denotes how well the phenotype can be predicted, given the genotype.