Expressivity refers to variations of a phenotype in genetics. The term is used to qualitatively characterize the variance of the phenotype. For example a qunatitative trait - like body height - might have large variance and therefore can make prediction of the phenotype difficult. Like penetrance in most instances expressivity is related to an allele or a mutation or more generally a genotype. In these instances expressivity denotes how well the phenotype can be predicted, given the genotype.
Notes
The wording "variable expressivity" usually means high expressivity. An internet search reveals many vague definitions and usage is not consistent in the literature.
See also:Penetrance
