Changes in chromosome number
Problems with cell division can produce cells with an abnormal number of chromosomes.
If this occurs during the formation of gametes, the fetus can have too few or too many chromosomes.
In most cases, having the wrong number of chromosomes is lethal to a human fetus, resulting in miscarriage. However there are exceptions.
Down's syndrome is a genetic disorder that causes severe intellectual disability. It is caused by an individual having three repeats (instead of two) of chromosome 21.
Having three copies of a chromosome is called trisomy.