Penetrance

Penetrance is a term used in genetics that describes the extent to which the properties controlled by a gene, its phenotype, will be expressed. Penetrance is the percentage of individuals given a specific genotype which actually express an associated phenotype.

A highly penetrant gene will express itself almost regardless of the effects of environment, whereas a gene with low penetrance will only sometimes produce the symptom or trait with which it has been associated. In some cases, the phenotype in question will occur only when the gene is present; in other cases, they may occur for unrelated reasons. In the case of low penetrance it is difficult to distinguish environmental from genetic factors.

Penetrance and heritability appear closely related at first glance, but in fact it is possible to carry a huge number of inherited genes with low penetrance and not be aware of them. The opposite is not the case however, if you carry a higher penetrant gene, you will know (assuming the gene has a noticeable effect, many do not).

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 explain – 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. Most genes make their little contribution to a very complex milieu of biological interactions, to which many other genes are also contributing.

As a result, most genes and their effects and mechanisms of action are very difficult to fully understand, because the required observations and experiments are complex and difficult to devise. Even if such observations and experiments were conducted, however, some theorists would still hold that because all traits are influenced by genetic factors as well as by non-genetic factors, no trait can be determined strictly by genes. As David S. Moore put it in The Dependent Gene (2002), "…the idea that some traits are more - or less – genetically determined than others makes about as much sense to [these theorists] as the idea that there are monkeys living inside of our heads."

The penetrance of some diseases is age-related, complicating the determination of true penetrance. 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.