Incomplete penetrance

Penetrance in genetics is the proportion of individuals carrying a particular variant (or allele) of a gene (the genotype) that also express an associated trait (the phenotype). In medical genetics, the penetrance of a disease-causing mutation is the proportion of individuals with the mutation who exhibit clinical symptoms. For example, if a mutation in the gene responsible for a particular autosomal dominant disorder has 95% penetrance, then 95% of those with the mutation will develop the disease, while 5% will not.

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%.

Common examples used to show degrees of penetrance are often highly penetrant. There are several reasons for this:

Penetrance can be difficult to determine reliably, even for genetic diseases that are caused by a single polymorphic allele. For many hereditary diseases, the onset of symptoms is age related, and is affected by environmental factors such as nutrition and smoking, as well as genetic cofactors and epigenetic regulation of expression:

A consensus definition of what constitutes the presence of a phenotype is essential for determining the penetrance of an allele. For hereditary hemochromatosis, a disease caused by excess intestinal iron absorption, the degree of penetrance has been a subject of controversy for many years and illustrates the challenges facing investigators seeking a quantitative measure of penetrance. Individuals who are homozygotes for the C282YA allele of the HFE gene are at risk for developing lethal concentrations of iron, particularly in the liver. Typically patients develop clinical disease in late-middle age.

...
Wikipedia