Down syndrome, also known as trisomy 21, is the most common identifiable form of mental retardation caused caused by an extra copy of chromosome 21 in the patient's set of chromosomes (karyotype). Nevertheless, this condition is also associated with other karyotypes as described bellow. Besides moderate to severe mental retardation, developmental abnormalities and other congenital conditions are also observed, such as heart disease (30-50% of patients), intestinal malformation (10-12%), visual or hearing impairment (50%), immunological deficiencies, and augmented predisposition to leukaemias and thyroid problems. Other characteristics include a broad short skull, broad hands with short fingers, small ears, small mouth, flattened nasal bridge, and varied degrees of altered (dysmorphic) body features. Down syndrome patients frequently develop Alzheimer's disease in early life stages. The average life expectancy of a Down's syndrome patient is about the age of 35.

In addition to trisomy 21, Down syndrome may be caused by either one of...