Down Syndrome

Down Syndrome
National Institute of Mental Health

What is Down Syndrome?

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. 

(For more information on chromosomes, see Cells 101.)

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