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.
Reciprocal translocation
Nondysjunction in maternal meiosis
Translocation defect
Nondysjunction in paternal meiosis