Down's syndrome is caused by the trisomy of which chromosome pair?

In man,the normal number of chromosomes is

An abnormal male child is born to normal parents. This child shows a short neck,stubby hands,slanting eyes,a permanently open mouth with a projecting upper lip,and short stature. This abnormality can be attributed to:

Example for autosomal hyper aneuploidy is

$21$ trisomy in humans causes

Down's syndrome in humans is due to