how does down syndrome caused?
 Down syndrome can be caused by one of three types of abnormal cell division involving chromosome 21. The three genetic variations include: Trisomy 21– More than 90% of Down syndrome cases are caused by trisomy 21. An extra chromosome (chromosome 21) originates in the development of either the sperm or the egg. When the egg and the sperm unite to form the fertilized egg, three (rather than two) chromosomes 21 are present. 