Penetrance is the probability that a person carrying a disease-associated genotype will develop the disease within a given time period. If you were to think about it as an equation, penetrance is the number of individuals displaying clinical features of a condition, divided by the number of individuals with a disease-causing mutation, multiplied by 100.
A disease is said to show incomplete or reduced penetrance when it has clearly been demonstrated that individuals who possess a disease-associated genotype show absolutely no manifestations of the condition. In this case, the number of individuals who display clinical features of the condition is less than those who carry the mutation.
A disease is said to show complete penetrance if a person carrying a disease associated genotype always develops the condition. In terms of our equation, the number of individuals who display clinical features of the condition is equal to those who carry the mutation.
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