Penetrance

The likelihood a given gene will result in disease. For example, if half (50%) of the people with the neurofibromatosis (NF) gene have the disease NF, the penetrance of the NF gene is 0.5.

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The frequency, expressed as a fraction or percentage, of individuals who are phenotypically affected, among persons of an appropriate genotype (i.e., homozygous or hemizygous for recessives, heterozygous or hemozygous for dominants); for an autosomal dominant disorder, if only a proportion of individuals carrying the mutant allele display the abnormal phenotype, the trait is said to show incomplete p.. If all with the mutant allele show the abnormal phenotype, the trait is said to have complete or full p.. See penetration.

- genetic p. (pen′e-trans) the extent to which a genetically determined condition is expressed in an individual.

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pen·e·trance 'pen-ə-trən(t)s n the proportion of individuals of a particular genotype that express its phenotypic effect in a given environment compare EXPRESSIVITY

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n.

the frequency with which the characteristic controlled by a gene is seen in the individuals possessing it. Complete penetrance occurs when the characteristic is seen in all individuals known to possess the gene. If a percentage of individuals with the gene do not show its effects, penetrance is incomplete. In this way a characteristic in a family may appear to 'skip' a generation.

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pen·e·trance (penґə-trəns) [L. penetrare to enter into] in genetics, the frequency of expression of a genotype under defined conditions. If it is less than 100 per cent, the trait is said to exhibit reduced penetrance or lack of penetrance.