the observed Down syndromes. [ 58 ] some of the cells in the body are normal and other cells have trisomy 21, this is called mosaic Down syndrome (46,XX/47,XX,+21). [ 59 ] [ 60 ] Trisomy 21 Trisomy 21 (47,XX,+21) is caused by a meiotic nondisjunction event. With nondisjunction, a gamete