Question

How ovum containing an extra chromosome number 21 occurs?

Answer 1

This occurs due to an error in meiosis. I assume you are familiar with the process? When the spindle fibres attach to the chromatids, they may be unevenly pulled to the poles, meaning there is an extra chromosome on one side. So, when the cell splits into 4 non identical gametes, one will have an extra chromosome.

Hope that helps!