Down&;s syndrome most commonly occurs due to: (A) Reciprocal translocation, (B) Nondysjunction in maternal meiosis, (C) Translocation defect, (D) Nondysjunction in paternal meiosis
Answer is B. 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.