In genetics, Penetrance is the percentage of people carrying a specific variant of a gene that also shows an associated trait. Penetrance of a disease-causing mutation is the percentage of people with the mutation who display clinical symptoms. For instance, if a mutation in the gene that results in a specific autosomal dominant disorder has 80 percent penetrance, then 80 percent of those with the mutation will have the disease, while 20 percent will not.
If clinical symptoms are shown in all people who have the disease-causing mutation, this condition is said to have complete penetrance. A condition that shows complete penetrance is referred to as neurofibromatosis type 1. In this case, the penetrance is 100 percent.
An allele is said to have incomplete penetrance if some people who have the disease-causing mutation do not express the characteristic even though they are carrying the allele. An autosomal dominant condition that shows incomplete penetrance, for example, is familial breast cancer.
If an allele has low penetrance, the trait that it expresses will not be obvious in an individual that carry the allele. An allele with high penetrance results in a trait that is almost always apparent. In low penetrance cases, it can be difficult to differentiate genetic from environmental factors.