penetrance

Suppose a hundred people all carry the same disease-causing genotype, yet only seventy of them ever develop the condition. Penetrance is that fraction — the proportion of individuals with a given genotype who actually show the associated trait. Here penetrance would be 70%.

Penetrance is an all-or-nothing measure: it asks only whether the trait appears at all, not how strongly. When every carrier of a genotype shows the trait, penetrance is complete (100%); when only some do, it is incomplete or reduced. The unaffected carriers still have the genotype and can pass it on.

Reduced penetrance is why a dominant condition can seem to 'skip' a generation — a carrier who never shows the trait still transmits the allele to a child who does. Penetrance can depend on age (some conditions only appear later in life), on sex, on other genes, and on environment, so the same genotype can have very different penetrance in different settings.