The proportion (in percent) of individuals with a particular gene combination that express the corresponding trait.
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 does not show its effects, penetrance is incomplete. In this way a characteristic in a family may appear to skip a generation.
The frequency of manifestation of a hereditary condition in individuals. In theory, if the genotype is present, penetrance should be 100%. That is not usually the case, as a result of the modifying effects of other genes.