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
Down's syndrome most commonly occurs due to - 
Select and explain. 
A. Reciprocal translocation, B. Nondysjunction in maternal meiosis, C. Translocation defect, D. Nondysjunction in paternal meiosis
B: Nondysjunction in maternal meiosis
Answer: B: Nondysjunction in maternal meiosis 
Explanation: 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.