Down&;s syndrome most commonly occurs due to
Down syndrome occurs due to trisomy 21. In 97% of cases it occurs as result of non dysfunction of chromosome during maternal meiosis. Reference : Nelson TB of pediatrics 19th edition pg 402.
Reciprocal translocation
Nondysjunction in maternal meiosis
Translocation defect
Nondysjunction in paternal meiosis
Genetic and genetic disorders