, Down's syndrome is a genetic disorder caused by the presence of an additional copy of chromosome number 21 (trisomy of 21). It was first described by Langdon Down in 1866 . While the term "Mongolian idiocy" is not used in the current text due to its offensive nature, historical context associates it with the physical features (like the palm crease and flat face) described for Down's syndrome. In contrast, Klinefelter's syndrome is 47, XXY, and Turner's syndrome is 45, XO .