Mendelian inheritance · USMLE Step 1

A pedigree shows affected individuals in every generation, but some individuals who carry the mutant allele do not express the phenotype. This is an example of:

  1. Variable expressivity
  2. Incomplete penetrance
  3. Genetic anticipation
  4. Codominance
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Correct answer: Incomplete penetrance

Incomplete penetrance means individuals with the mutation may or may not express the phenotype. Variable expressivity affects severity, not presence. Anticipation involves earlier onset over generations.

Difficulty: Medium Question 14 of 20

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