Nevertheless, 95 % of those who are afflicted with Down's do not get it genetically, as the chromosomes of the parents are normal; rather there is an abnormal division at the time the sperm joins the egg, so that the fetus has one more of the 21st chromosome than ordinary people.
然而，百分之九十五的唐氏症患者並非由遺傳而來，父母染色體均正常，但在精子、卵子結合時產生不正常的分裂，造成胎兒第廿一號染色體較常人多一個。