This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetic and genetic disorders, 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: Down syndrome occurs due to trisomy 21. In 97% of cases it occurs as result of non dysfunction of chromosome during maternal meiosis. Reference : Nelson TB of pediatrics 19th edition pg 402.