Down syndrome is a disorder caused by the abnormal extra copy of the 21st chromosome. Because of the presence of this extra chromosome, several abnormalities may be produced in the individual: most commonly a subnormal intelligence, defects in the heart, abnormalities of intestine, childhood leukemia, early Alzheimer’s disease etc.
The degree of mental retardation is variable – however, these children are trainable to a very large extent. More information on Down syndrome can be found here
It is possible to a large extent to detect Down syndrome before birth. But detection is not 100% at present.