National Institute of Child Health and Human Development
National Institute of Child Health and Human Development
NICHD Information Resource Center
P.O. Box 3006
Rockville, MD 20847
Phone: 1-800-370-2943
Fax: 301-984-1473

Down syndrome is set of mental and physical symptoms that result from having an extra copy of Chromosome 21.
Normally, a fertilized egg has 23 pairs of chromosomes. In most people with Down syndrome, there is an extra copy of Chromosome 21 (also called trisomy 21 because there are three copies of this chromosome instead of two), which changes the body’s and brain’s normal development.

Even though people with Down syndrome may have some physical and mental features in common, symptoms of Down syndrome can range from mild to severe. Usually, mental development and physical development are slower in people with Down syndrome than in those without the condition.
Mental retardation is a disability that causes limits on intellectual abilities and adaptive behaviors (conceptual, social, and practical skills people use to function in everyday lives). Most people with Down syndrome have IQs that fall in the mild to moderate range of mental retardation. They may have delayed language development and slow motor development
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