Penetrance is a term use in genetics describing the proportion of individuals carrying a particular variation of a gene (an allele or genotype) that also express a particular trait (the phenotype). For example, known mutations in the gene responsible for Huntington disease have 95% penetrance, whereby 5% of those with the dominant allele for Huntington disease don't develop the disease and 95% do. In other word, penetrance is the percentage of individuals with a specific genotype that possess an associated phenotype. For example, if 50% of the individuals carrying the "blue" allele are blue, the "blue" allele has 50% penetrance. Penetrance only considers whether individuals express the trait or not — for variation in the degree of expression of a given trait, see expressivity.