<prompt> Downs syndrome most commonly occurs due to -Reciprocal translocationNondysjunction in maternal meiosisTranslocation defectNondysjunction in paternal meiosis  <response>: 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. 