Penetrance in genetics is the proportion of individuals carrying a particular gene variant (allele or genotype) that also possess an associated trait (phenotype). If penetrance of a disease allele is 100%, then all individuals carrying that allele will have the associated disease. Penetrance only considers whether individuals express the trait or not. This differs from expressivity, which characterizes qualitatively or quantitatively the extent of phenotypic variation given a particular genotype. Penetrance is age related (Bessett et al., 1998) and is affected by environmental and behavioral factors such as diet and smoking. It is also modified by other genes and epigenetic regulation.