<prompt> Down syndrome most commonly occurs due to ___Reciprocal translocationNon-disjunction in maternal meiosisMosaicismNon-disjunction in paternal meiosis  <response>: Down;s syndrome occurs due to nondisjunction in maternal meiosis. Down;s syndrome: Most common chromosomal disorder Incidence: 1800 live bihs Mechanism: Trisomy 21 meiotic maternal nondisjunction 95 Robesonian translocation: 4 Mosaic pattern: 1 Non-disjunction points to the failure of the homologous chromosome to seperate during the meiotic division. Ref: Nelson textbook of pediatrics 21st edition Pgno: 658 