Down's syndrome most commonly occurs due to -

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.