APA Dictionary of Psychology

n. in genetics, the extent to which the effects of an allele are manifest in the individuals possessing it, expressed as the fraction or percentage of individuals carrying that allele who manifest the trait associated with it. If all persons who possess a particular dominant allele develop the associated trait, the allele is said to show complete penetrance (100%). In contrast, incomplete penetrance occurs when some individuals with a particular allele do not develop the associated trait.