Definition of Penetrant trait

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Penetrant trait: A genetic trait (characteristic) that is present in the genome and manifests itself in the individual. By contrast, a nonpenetrant trait is one that, although present, does not manifest itself.

If, for example, 100 people have a particular genetic trait but only 80 of them express it, the penetrance of that trait is 80%. The trait is penetrant and expresses itself in 80% of people who have it.