A 32-year-old woman has a child with Down syndrome. Karyotyping of the child reveals a Robertsonian translocation involving chromosome 21 attached to chromosome 14, with no free chromosome 21. The mother's karyotype is normal. What is the approximate recurrence risk for Down syndrome in her subsequent pregnancies?

• A 50%
• B 25%
• C 10 to 15%
• D 1% ✓

Explanation

When a child has de novo Robertsonian translocation Down syndrome and both parents have normal karyotypes, the recurrence risk is approximately 1%, slightly above the general population risk. D 10 to 15% recurrence risk applies only when a parent is a balanced carrier of the Robertsonian translocation. The 25% and 50% figures apply to Mendelian disorders, not chromosomal translocations.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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