This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pediatrics
Which mutation is seen in Down’s syndrome - 
Select and explain. 
A. Monosomy X, B. Trisomy 21, C. Trisomy 22, D. Trisomy 22
B: Trisomy 21
Answer: B: Trisomy 21 
Explanation: 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