A newborn with hypotonia, single palmar crease, flat facial profile, upward slanting palpebral fissures, and a large tongue is suspected to have trisomy 21. The karyotype shows 46 chromosomes with a derivative chromosome between 14 and 21. What is the mechanism and the recurrence risk implication?
- A Mosaicism; recurrence risk similar to age-related risk
- B Robertsonian translocation; if parent is a carrier, recurrence risk up to 100% if mother carries t(21;21)
- C Robertsonian translocation t(14;21); if mother is a carrier, recurrence risk ~10–15% ✓
- D Isochromosome 21q; recurrence risk negligible
Explanation
A karyotype of 46 chromosomes with a derivative chromosome between 14 and 21 indicates Robertsonian translocation t(14;21) — this is not free trisomy 21 (which shows 47 chromosomes). In Robertsonian translocation Down syndrome, the extra chromosome 21 is fused to chromosome 14. Parental karyotyping is essential: if a parent carries the translocation, recurrence risk is much higher than age-related risk. If the MOTHER is the carrier, empirical recurrence risk is approximately 10–15%; if the FATHER is the carrier, approximately 2–5%. However, if one parent carries a t(21;21) Robertsonian translocation (both chromosome 21 long arms fused), ALL offspring will have Down syndrome (100% recurrence). Mosaicism shows 46 and 47 chromosome clones, not a derivative chromosome.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.