Penetrance is a term used in genetics that describes the extent to which the properties controlled by a gene, its phenotype, will be expressed.

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 qualities in question will only occur when the gene is present; in other cases, they may occur for unrelated reasons. In the latter case, it will be more difficult to distinguish environmental from genetic factors, or even to detect that there is a genetic contribution at all.

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.

Most of the genes that are used as examples in genetics textbooks are highly penetrant, and the symptoms that they are associated with are highly heritable. There are several related reasons for this:

1. highly penetrant genes, and highly heritable symptoms, are easier to explain – if you’ve got the gene, you’ve got the thing (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 are so strong in their effects. Most genes make their little contribution to a very complex milieu of biological interactions, to which many other genes are also contributing.

This can mean either (depending on where you're coming from) that most genes and their effects and mechanisms of action are very difficult fully to understand, because the required observations and experiments are complex and difficult to devise, or else that "…the idea that some traits are more - or less – genetically determined than others makes about as much sense to us as the idea that there are monkeys living inside of our heads." (David Moore, The Dependent Gene, 2001).