CBSE Class 9 Answered

There are several causes for chromosomal disorders. Most of these occur in the foetus itself. Changes in chromosome number can occur during sperm or egg formation or during the early development of embryo. Most chromosome abnormalities occur as an accident in the egg or sperm. So when there is a mistake during production of sperm and egg cells in meiosis, chromosomal disorders can result. The age of mothers is especially important in avoiding chromsomal abnormalities. So aging of sperm and egg cells is another cause. Also habits of mothers during pregnancy are also important in this respect.

Other possible causes of chromosomal disorders are exposure to ionizing radiations, viral infections and chemical toxins.