Down syndrome is a genetic disorder caused by an extra copy of chromosome 21. Typically, a baby is born with 46 chromosomes, but babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. This extra genetic material alters the course of development and causes the characteristics associated with Down syndrome. The extra chromosome 21 leads to the physical features and developmental challenges that can occur among people with Down syndrome. Down syndrome is also referred to as Trisomy 21.

The extra chromosome 21 in Down syndrome is caused by a random error in cell division called nondisjunction. This error results in the presence of an extra copy of chromosome 21. The majority of the time, the extra copy of chromosome 21 comes from the mother in the egg. In a small percentage (less than 5%) of cases, the extra copy of chromosome 21 comes from the father through the sperm. Sometimes, a parent who does not have Down syndrome may carry a translocation in chromosome 21 that can be passed on to children and cause Down syndrome.

In summary, Down syndrome is caused by an extra copy of chromosome 21, which is the result of a random error in cell division called nondisjunction.