Down's syndrome most commonly occurs due to –
Reciprocal translocation
Nondysjunction in maternal meiosis
Translocation defect
Nondysjunction in paternal meiosis
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.