The proportion of individuals with a mutation causing a particular disorder who exhibit clinical symptoms of that disorder.
A condition, most commonly inherited in an autosomal dominant manner, is said to show complete penetrance if clinical symptoms are present in all individuals who have the disease-causing mutation. A condition which shows complete penetrance is neurofibromatosis type 1 - every person who has a mutation in the gene will show symptoms of the condition. The penetrance is 100%.
Some conditions are described as having reduced or incomplete penetrance. This means that clinical symptoms are not always present in individuals who have the disease-causing mutation. An example of an autosomal dominant condition showing incomplete penetrance is familial breast cancer due to mutations in the BRCA1 gene. Females with a mutation in this gene have an 80% lifetime risk of developing breast cancer. The penetrance of the condition is therefore 80%.
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