Penetrance

Overview
Penetrance is a term used in genetics that describes the extent to which the properties controlled by a gene, its phenotype, will be expressed. 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. Penetrance is the percentage of individuals with a specific genotype which possess an associated phenotype. For example, if 50% of the individuals carrying the "blue" gene are blue, the "blue" gene has 50% penetrance.

If a gene is highly penetrant, the trait it produces will always or almost always be apparent in an individual carrying the gene. A gene 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:
 * 1) Highly penetrant genes, and highly heritable symptoms, are easier to demonstrate – if the gene is present, the phenotype is expressed (recessivity, dominance, and co-dominance are fairly simple additions to this principle);
 * 2) Genes which are highly penetrant are more easily noticed by geneticists, and genes for symptoms which are highly heritable are more easily inferred to exist, and then more easily tracked down.

However, relatively few of the genes in the genome show high penetrance. Traits such as height or intelligence are modified 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.