Penetrance

Penetrance in genetics is the proportion of individuals carrying a particular variant (or allele) of a gene (genotype) that also expresses an associated trait (phenotype). In medical genetics, the penetrance of a disease-causing mutation is the proportion of individuals with the mutation that exhibit clinical symptoms among all individuals with such mutation.^[1] For example: If a mutation in the gene responsible for a particular autosomal dominant disorder has 75% penetrance, then 75% of those with the mutation will go on to develop the disease, showing its phenotype, whereas 25% will not.  

Penetrance only refers to whether an individual with a specific genotype exhibits any phenotypic signs or symptoms, and is not to be confused with variable expressivity which is to what extent or degree the symptoms for said disease are shown (the expression of the phenotypic trait). Meaning that, even if the same disease-causing mutation affects separate individuals, the expressivity will vary.^{[1][2] [3]}