Genetic mutations can cause disease, but not all carriers show symptoms. The proportion of symptomatic carriers is called penetrance. Complete penetrance means the gene always causes disease, while incomplete penetrance means not all carriers develop symptoms. Coexisting genes can modify disease expression.
In genetics, some genetic mutations are capable of causing disease. Sometimes, all people with a mutation show symptoms of that disease, and sometimes, only some of the people who carry the mutation show symptoms. The proportion of people who show symptoms of the mutation is called the penetrance of the gene.
If a person carries a gene associated with a medical condition, it does not necessarily mean that the gene will cause the disease. In addition to gene action, other parameters, such as environmental stresses and the coexistence of other interacting genes, can alter the development of medical conditions. On the other hand, the presence of some genes means that the disease will always occur.
Complete penetrance is the scientific term for when the presence of a gene always causes disease. An example of complete penetrance is the condition neurofibromatosis type 1. This means that 100% of people who have the genetic mutation associated with the disease show 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 fully penetrant medical condition. Although the presence of the mutated gene causes clinically visible symptoms, the signs can vary widely in severity from person to person. The most obvious cases have bone abnormalities or problems with the circulatory system, but mild cases may show only minor, almost imperceptible signs, such as long fingers and 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, while others have 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 of the mutated gene and prevent the expression of the unhealthy trait.
Incomplete penetrance refers to mutations that have less than 100% penetrance. One example is a mutation in the BRCA1 gene, which is associated with breast cancer. This genetic mutation means that around 80% of women with the mutation will develop breast cancer at some point in their lives, but 20% will not. Retinoblastoma, a cancer of the eye, also has a genetic cause, although the mutation doesn’t always cause cancer.
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