How does someone with Down syndrome get that extra chromosome?

People with Down syndrome have an extra copy of chromosome 21. There are a few different ways that this can happen, but the most common is called nondisjunction. Nondisjunction occurs when there is an error in cell division, resulting in cells that have either too many or too few chromosomes. This can happen either in the egg or the sperm cell, and it can also happen during early embryonic development. If the developing embryo has an extra copy of chromosome 21, the fetus will have Down syndrome.