Penetrance and expressivity

Not all patients with the same gene mutation (i.e. genotype) exhibit the same degree of observable characteristics (i.e. phenotype). This phenomenon is described using two terms - penetrance and expressivity. Penetrance describes the proportion of a population of individuals who carry a disease-causing allele who express the related disease phenotype. Expressivity describes the extent to which a genotype shows its phenotypic expression in an individual.

• Modifier genes
• Environmental factors
• Allelic variation

• describes 'how likely' it is that a condition will develop
• examples of conditions with incomplete penetrance include retinoblastoma and Huntington's disease
• in contrast, achondroplasia shows 100%, or complete, penetrance

• describes the 'severity' of the phenotype
• a condition with a high level of expressivity is neurofibromatosis