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Trisomy 21 nondisjunction — MRCPCH TAS MCQ

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EasyGeneticsTrisomy 21 nondisjunctionMRCPCH TAS

A newborn has hypotonia, an upslanting palpebral fissure and a complete atrioventricular septal defect. Karyotype shows 47,XX,+21. What is the most likely explanation?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: E — Meiotic nondisjunction

Free trisomy 21 is most commonly due to meiotic nondisjunction. A balanced parental translocation is important to consider in translocation Down syndrome, but the karyotype here shows an additional free chromosome 21. The pearl is that recurrence counselling differs between free trisomy and Robertsonian translocation.

Reference: Nelson Textbook of Pediatrics; RCPCH Theory examination syllabi; Lissauer's Illustrated Textbook of Paediatrics