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Penetrance

Penetrance is a term used in genetics describing the proportion of individuals carrying a particular variation of a gene (an allele) that also express a particular trait (the phenotype). For example, Huntingtons disease has 95% penetrance, whereby 5% of those with the dominant allele for Huntingtons don't acquire the disease and 95% do. In other word, penetrance is the percentage of individuals with a specific genotype that possess an associated phenotype. For example, if 50% of the individuals carrying the "blue" allele are blue, the "blue" allele has 50% penetrance. Penetrance only considers whether individuals express the trait or not — for variation in the degree of expression of a given trait, see expressivity.

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If a allele is highly penetrant, the trait it produces will always or almost always be apparent in an individual carrying the allele. Reduced or incomplete penetrance occurs when some individuals fail to express the trait, even though they carry the allele. An allele with low penetrance will only sometimes produce the symptom or trait with which it has been associated at a detectable level. In the case of low penetrance it is difficult to distinguish environmental from genetic factors.

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

Highly penetrant alleles, and highly heritable symptoms, are easier to demonstrate – if the allele is present, the phenotype is expressed (recessivity, dominance, and co-dominance are fairly simple additions to this principle);
Alleles which are highly penetrant are more easily noticed by geneticists, and alleles for symptoms which are highly heritable are more easily inferred to exist, and then more easily tracked down.

However, relatively few of the alleles in the genome show high penetrance. Traits such as height or intelligence are influenced by multiple genes as well as by environmental factors. Distribution of polygenic traits often falls along a bell curve.

The penetrance of some diseases is age-related. An example is multiple endocrine neoplasia 1 (MEN 1), a disorder characterized by parathyroid hyperplasia and pancreatic islet-cell and pituitary adenomas. It is due to a mutation in the menin gene on chromosome 11q13. In one study the age-related penetrance of MEN1 was 7 percent by age 10 years and nearly 100 percent by age 60 years.