Penetrance refers to the proportion of individuals with a particular genotype (genetic makeup) able to exhibit the corresponding phenotype (observable traits or characteristics).
In other words, it is the likelihood that a genetic variant will be expressed (as a particular trait or disease).
Penetrance is expressed as a percentage. The percentage represents the proportion of individuals with a particular genotype who actually show the expected phenotype.
For example: If a particular genetic variant has 80% penetrance, it means that 80% of individuals with that variant will show the associated trait or disease.
What determines genetic penetrance?
Several factors, including genetic and environmental factors, can influence penetrance.
In some cases, other genetic variants or environmental factors may modify the expression of the trait associated with a particular genotype, resulting in incomplete or variable penetrance. In other cases, individuals with the same genotype may show different degrees of penetrance.
Types of Genetic Penetrance
There are two main types of genetic penetrance: complete penetrance and incomplete (or variable) penetrance.
Complete penetrance:
In this condition, all individuals who inherit the disease-causing mutation will show clinical symptoms of the disease.
This means that if a person has a disease-causing mutation, they will develop the disease without any exceptions. Complete penetrance is commonly observed in diseases caused by mutations in a single gene, such as Huntington’s disease, Sickle cell disease, etc.
Incomplete penetrance:
Incomplete or variable penetrance: Only a subset of individuals who inherit the disease-causing mutation will show clinical symptoms of the disease. In this case, some individuals with the disease-causing mutation may develop the disease, while others may not.
Incomplete penetrance is often observed in diseases caused by a combination of genetic and environmental factors, such as breast cancer or diabetes.
Examples of Penetrance:
Polydactyly:
Polydactyly is a condition in which an individual has extra fingers or toes. This condition can be caused by mutations in several genes, including GLI3 and SHH. However, the penetrance of these mutations can vary, with some individuals showing extra digits and others not.
BRCA1 gene and breast cancer:
Mutations in the BRCA1 gene are associated with an increased risk of developing breast and ovarian cancer. However, not all individuals with a mutation in this gene will develop cancer. The penetrance of BRCA1 mutations is estimated to be around 60-70%, meaning that 60-70% of individuals with a mutation in this gene will develop breast cancer.
Huntington’s disease:
Huntington’s disease is a rare, inherited neurological disorder caused by a mutation in the huntingtin (HTT) gene. The penetrance of the HTT mutation is nearly complete, meaning that almost all individuals with the mutation will develop symptoms of the disease at some point in their lives.
Hereditary hemochromatosis:
Hereditary hemochromatosis is a genetic disorder in which the body absorbs too much iron from food. This leads to a buildup of iron in the organs.
Mutations in the HFE gene are the most common cause of hereditary hemochromatosis. However, not all individuals with a mutation in this gene will develop the disorder.
The penetrance of HFE mutations is estimated to be around 50%, meaning that 50% of individuals with a mutation in this gene will develop the disorder.
It is important to note that penetrance can vary between families, and even between individuals within a family.
Factors that can influence penetrance include genetic background, environmental factors, and lifestyle choices.
In some cases, genetic counseling and testing may be recommended for individuals with a family history of a genetic condition. It is always better to learn about the risks of developing the disease and make informed decisions about medical management and family planning.