Pedigree analysis · USMLE Step 1

Which clue argues against X-linked dominant inheritance in a pedigree?

  1. An affected male has all affected daughters and no affected sons
  2. The trait appears in every generation without ever skipping
  3. Generations are skipped: unaffected parents have an affected child
  4. Affected females clearly outnumber affected males overall
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Correct answer: Generations are skipped: unaffected parents have an affected child

X-linked dominant traits rarely skip generations. Two unaffected parents producing an affected child indicates a recessive-type pattern, arguing against X-linked dominant inheritance. The other options are all consistent with X-linked dominant.

Difficulty: Medium Question 5 of 19

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