Which mutation is seen in Down’s syndrome -
The basic molecular defect in Down's syndrome is the extra genetic material of chromosome 21. It may be due to :

1) Trisomy 21

This is the most common cause (95%). There is an extra chromosome 21 (i.e. three 21 chromosomes) which is due to meiotic nondisjunction in the ovum. The extra chromosome is of maternal origin. The most important risk factor is advanced maternal age (> 35 years).

2) Roberston translocation

This accounts for 3% of cases. Extra material of 21 chromosomes comes from Robertson translocation of the long arm of chromosome 21 to another acrocentric chromosome (22, or 14 or 15), i.e. t (22: 21), t (14:21), or t (15:21). It has no relation with maternal age.

3) Mosaicism (Mosaic 21)

In 2% of patients, there is mosaicism, i.e. occurrence of two or more different types of the cell population (46 XX / 47 XX + 21) in the same individual, due to mitotic nondisjunction of chromosome 21. There is no correlation with maternal age.

4) Partial trisomy

It is very very rare. The long arm of chromosome 21 triplicates
Monosomy X
Trisomy 21
Trisomy 22
Trisomy 22