fundamentals
Inheritance of Single Gene Disorders
Penetrance and expressivity
Penetranceis defined as the percentage of people who have the allele (the specific form of a gene responsible for the variations in which a given trait can be expressed) and who develop the corresponding phenotype (trait). For example, if half the people with a allele show its trait, its penetrance is 50%. Penetrance may be complete or incomplete. A gene with incomplete penetrance is not always expressed even when the trait it produces is dominant or when the trait is recessive and present on both chromosomes. Penetrance of a given gene may vary from person to person and may depend on a person’s age. Even when a specific allele is not expressed (nonpenetrance), the unaffected carrier of the allele can pass it to their children, who may have the trait. Expressivityrefers to how much a trait affects a single person, that is, whether the person is greatly, moderately, or mildly affected. People who have the same gene may be affected differently. Two terms explain these differences: 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 is complete (100%) if everyone with the gene has the trait. Penetrance is incomplete if only some people with the gene have the trait. For example, 50% penetrance means that only half the people with the gene have the trait. Expressivity refers to how much the trait affects (is expressed in) a person. A trait may be very pronounced, barely noticeable, or in between. 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 varies. 