A person with Down's syndrome has an extra copy of one of the chromosomes, which results in mental retardation, as well as other symptoms.

Chromosomes contain genetic material that directs the actions of cells. Normally, human cells have 23 pairs of chromosomes, resulting in 46 chromosomes. One of each pair comes from the mother, and one of each pair comes from the father. Down's syndrome describes the symptoms and signs displayed by someone who has an extra 21st chromosome, which results in 47 total chromosomes. In the US, about 1 out of 1000 infants is born with Down's syndrome.