How you get it: Down's Syndrome is usually not inherited. The only form of this condition that can be inherited is called Translocation Down's Syndrome. Only about four percent of children that have Down's syndrome have translocation. He or she can pass on the translocation to the child, who will and up having an extra copy of Chromosome 21, which is what Down's Syndrome is.
When translocations are inherited, the mother or father has some rearranged genetic material, but no extra genetic material — this means he or she is a balanced carrier. A balanced carrier has no signs or symptoms of Down syndrome, but he or she can pass the translocation on to children, causing extra genetic material from chromosome 21.