Down syndrome is by far the most common and best known chromosomal disorder in humans and the most common cause of intellectual disability.
It is characterized by mental retardation, dysmorphic facial features, and other distinctive phenotypic traits. Down syndrome is primarily caused by trisomy of chromosome 21; this is the most common trisomy among live births. The term mongolism was once commonly used but is now considered obsolete.
Like most diseases associated with chromosomal abnormalities, trisomy 21 gives rise to multiple systemic complications as part of the clinical syndrome. This chromosomal anomaly leads to both structural and functional defects in patients with Down syndrome. However, not all defects occur in each patient; there is a wide range of phenotypic variation.
*Sources: From Medscape Reference, http://emedicine.medscape.com/article/943216-overview