Penetrance is a term that describes the proportion of individuals, possessing a particular genotype, that display the phenotype expected of that genotype. If only 80% of the individuals of a genotype show the expected phenotype, then the gene is said to be 80% penetrant (most genes are fully penetrant).

Penetrance is distinct from expressivity; the penetrance of a gene refers only to whether it is expressed phenotypically or not, while expressivity refers to the degree of expression (so an allele may have high penetrance in a population, but varying levels of expressivity between individuals). Penetrance is often difficult to determine because the expression of an allele often depends heavily on epigenetic or environmental factors, such as smoking, drinking and nutrition.