Down syndrome refers to the type of mental retardation caused by a chromosomal aberration (chromosome 21) and involving characteristic physical appearance. Down Syndrome is also known as Trisomy 21.
Other /More definition:
Down syndrome refers to a chromosomal abnormality in which there are three (3) 21st chromosomes rather than the normal two (2) . Children with Down syndrome typically function at the moderate level of mental retardation, have an increased likelihood of medical problems, and have unusual physical features. This syndrome is also called Trisomy 21.

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