Expressivity refers to what aspect of a genetic disorder?

• A. Whether the trait is present or absent.
• B. The range and severity of features among affected individuals.
• C. The chance of being affected.
• D. The age at onset.

Answer: (B)

Expressivity refers to how differently a genetic change can manifest in people who have it. It describes the range and severity of clinical features among those affected by the same mutation—some may have mild signs, others more severe or additional features. This is distinct from penetrance, which is about the chance that the trait appears at all in carriers (complete vs incomplete). Age at onset is a separate consideration, describing when symptoms first appear rather than how variable the features are once present. A classic example is neurofibromatosis type 1, which shows variable expressivity: the same mutation can cause anything from a few café-au-lait spots to numerous neurofibromas and other problems.