Pedigree analysis · USMLE Step 1

Which mode of inheritance is nearly ruled out if affected males are found in consecutive generations and pass the trait to their sons?

  1. Autosomal dominant
  2. Autosomal recessive
  3. X-linked dominant
  4. Y-linked
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Correct answer: X-linked dominant

Male-to-male (father-to-son) transmission argues against X-linked inheritance, because a father passes his Y — not his X — to sons. Autosomal and Y-linked patterns all permit father-to-son transmission, so X-linked dominant is the one nearly ruled out.

Difficulty: Medium Question 10 of 19

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