This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
[{'content': "Down's syndrome most commonly occurs due to - \nSelect and explain. \nA. Reciprocal translocation, B. Nondysjunction in maternal meiosis, C. Translocation defect, D. Nondysjunction in paternal meiosis", 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Nondysjunction in maternal meiosis In trisomy 21, the additional number 21 chromosome is derived from the mother in 95% of cases. It is due to nondysjunction of 21st chromosome at the time of meiosis. In nondysjunction, chromosomal pair fails to separate in first meiotic division during gametogenesis.", 'role': 'assistant'}]