A 38-year-old woman delivers a baby with flat facies, upslanting palpebral fissures, epicanthal folds, single palmar crease, and an echogenic intracardiac focus on antenatal scan. Karyotype shows free trisomy 21. The most common mechanism producing this karyotype is:
- A Paternal meiotic nondisjunction in meiosis II
- B Maternal meiotic nondisjunction in meiosis I ✓
- C Postzygotic mitotic nondisjunction forming a mosaic
- D Robertsonian translocation between chromosomes 14 and 21
Correct answer: B. Maternal meiotic nondisjunction in meiosis I
Explanation
About 95% of Down syndrome cases are free trisomy 21 from meiotic nondisjunction of chromosome 21, and roughly 90 to 95% of these occur during maternal gametogenesis, predominantly in meiosis I when homologous chromosomes fail to separate. The risk rises sharply with maternal age above 35 years. B Robertsonian translocation accounts for only about 4% of cases, and mitotic error causing mosaicism accounts for about 1%.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.