In genetics, certain gene mutations are capable of causing disease. Sometimes, everybody with a mutation displays symptoms of that disease, and sometimes, only some of the people carrying the mutation display symptoms. The proportion of people who show symptoms from the mutation is called the penetrance of the gene.
If a person carries a gene that is associated with a medical condition, it doesn't necessarily mean that the gene will cause the illness. As well as the action of the gene, other parameters, such as environmental stresses and the coexistence of other, interactive genes, can alter the development of medical conditions. On the other hand, the presence of some genes means that the illness will always occur.
Complete penetrance is the scientific term for when the presence of a gene always results in disease. One example of complete penetrance is the condition neurofibromatosis type 1. This means that 100 percent of people who have the genetic mutation associated with the disease show its symptoms. Another example is a certain mutation of a particular gene called FGFR3, which always causes the carrier to have achondroplasia and show clinical signs of dwarfism.
Marfan Syndrome is another medical condition with complete penetrance. Although the presence of the mutated gene does result in clinically visible symptoms, the signs can vary widely in severity from person to person. More obvious cases have bone abnormalities or problems with the circulatory system, but mild cases may only display minor, almost unnoticeable signs, like long fingers and a tall stature.
Reduced or incomplete penetrance means that not all people with a genetic mutation develop signs of the associated disease. Some mutations have a high likelihood of symptoms, whereas others have a low penetrance. The reasons behind the absence of disease in some people, and the development of problems in other people, are not very well understood. Research into the involvement of coexisting genes in disease development, however, has found that, in some cases, other genes act as modifiers on the mutated gene and prevent the unhealthy trait from being expressed.
Incomplete penetrance refers to mutations that have less than 100 percent penetrance. One such example is a mutation of the BRCA1 gene, which is associated with breast cancer. This gene mutation means that about 80 percent of women with the mutation will develop breast cancer at some point in their lives, but 20 percent will not. Retinoblastoma, a cancer of the eye, also has a genetic cause, although the mutation does not always result in cancer.