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. 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.