A couple has one child with Down syndrome. The father's karyotype reveals a balanced Robertsonian translocation between chromosomes 14 and 21 (45,XY,der(14;21)(q10;q10)). Which statement about recurrence is correct?
- A Gametes can carry the der(14;21) plus a normal chromosome 21, producing translocation Down syndrome ✓
- B Recurrence risk equals the general age-related population risk
- C Robertsonian carriers cannot have phenotypically normal children
- D The translocation arose de novo in the affected child, so recurrence risk is negligible
Correct answer: A. Gametes can carry the der(14;21) plus a normal chromosome 21, producing translocation Down syndrome
Explanation
B carrier of a balanced rob(14;21) has 45 chromosomes but a normal phenotype. During gametogenesis, segregation can produce gametes containing the derivative chromosome together with a normal chromosome 21; fertilization by a normal gamete yields 46 chromosomes with trisomy of 21q material, that is, translocation Down syndrome. Because the father carries it here, recurrence risk is appreciable, roughly 1 to 3 percent for male carriers and higher for female carriers, far above the age-related population risk.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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