Q.Down's syndrome is due to

Down's syndrome is caused by non-disjunction during gamete formation, resulting in an extra copy of chromosome 21 (trisomy 21).

Down's syndrome arises when, during gametogenesis (usually oogenesis), chromosome 21 fails to separate properly at anaphase — a phenomenon called non-disjunction. This produces a gamete with an extra copy of chromosome 21. When such a gamete fuses with a normal gamete at fertilisation, the resulting individual has three copies (trisomy) of chromosome 21 instead of the usual two, giving a total karyotype of 47 chromosomes. …