Pedigree analysis · USMLE Step 1

In a pedigree for an X-linked recessive disease, which of the following patterns is most suggestive?

Both sexes affected about equally in every generation
Only females affected, with all males remaining unaffected
Mostly males affected, with no father-to-son transmission
All daughters of an affected father are themselves affected

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Correct answer: Mostly males affected, with no father-to-son transmission

X-linked recessive disorders affect males more often, because males are hemizygous for the X chromosome. A key clue is the absence of male-to-male transmission, since fathers pass a Y (not their X) to sons.

Difficulty: Medium Question 4 of 19

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