Definition
The proportion of individuals with a particular genotype who express the associated phenotype.
Use in clinical context
There are very few genotypes that always result in the associated phenotype because genes do not act in isolation. There are other genes influencing phenotypes, and environments also contribute to phenotypes.
Penetrance can be 100%, known as complete penetrance, as is the case in Huntingdon disease. Other disease-causing variants have high, but incomplete penetrance, for example some familial hypercholesterolaemia variants have penetrance of more than 90%. Other variants are important causes of disease but have lower penetrance, for example BRCA2 gene variant penetrance can be as low as 47% depending on the population.
