<prompt> Downs syndrome most commonly occurs due toReciprocal translocationNondysjunction in maternal meiosisTranslocation defectNondysjunction in paternal meiosis  <response>: In trisomy 21, the additional number 21 chromosome is derived from the mother in 95 of cases.It is due to nondisjunction of the 21st chromosome at the time of meiosis.In nondisjunction, chromosomal pair fails to separate in the first meiotic division during gametogenesis. 