Mendelian inheritance · USMLE Step 1

A patient with an autosomal dominant condition displays variable expressivity. Which best describes this phenomenon?

  1. The same mutation produces differing severity among individuals
  2. The trait skips generations before reappearing in descendants
  3. Multiple different genes independently produce the same phenotype
  4. The trait is expressed only in homozygous affected individuals
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Correct answer: The same mutation produces differing severity among individuals

Variable expressivity means the same genotype produces a range of phenotypic severities. Penetrance concerns whether the trait is expressed at all, not its degree. Skipping generations is a hallmark of recessive inheritance.

Difficulty: Medium Question 10 of 20

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