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How genotype is translated into phenotype depends on penetrance and expressivity.

Penetrance refers to whether the gene is expressed or not. That is, it refers to how many people with the gene have the trait associated with the gene. Penetrance may be complete (100%) or incomplete (eg, 50% when only half the people have the trait).

Expressivity determines how much the trait affects or how many features of the trait appear in the person. It ranges from complete to minimal, or it may not be present. Various factors, including genetic makeup, exposure to harmful substances, other environmental influences, and age, can affect expressivity.

Both penetrance and expressivity can vary: People with the gene may or may not have the trait and, in people with the trait, how the trait is expressed can vary.