The only kind that can be transmitted by the parents is the "translocational" type. This type accounts for about 4 % of Down's syndrome sufferers. The cause is that the structure of the 21st chromosome divides, and an additional 21st attaches itself to another chromosome when the sperm and egg are joined.
唯一可能經由父母遺傳而來的是「轉位型」，此型約佔唐氏症患者的百分之四，成因是第廿一號染色體結構發生缺陷斷裂，轉移沾黏到其他染色體上去。