A couple has a child with translocation Down syndrome: 46,XX,der(14;21)(q10;q10),+21. Parental karyotyping reveals that the father carries the balanced Robertsonian translocation rob(14;21). What is the theoretical recurrence risk for Down syndrome in their offspring?
- A 1%
- B 10 to 15% ✓
- C 25%
- D 33%
Correct answer: B. 10 to 15%
Explanation
When a parent is a balanced carrier of a Robertsonian translocation involving chromosome 21, the empirical recurrence risk for Down syndrome is about 10 to 15% if the mother is the carrier and about 3 to 5% if the father is the carrier. The commonly cited figure of 10 to 15% applies to maternal carriers. For paternal carriers the risk is lower, but among the given options 10 to 15% is the best answer as the key concept being tested is that a balanced carrier parent confers a substantially increased recurrence risk, not the low de novo figure of 1%.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.