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Down syndrome
Background
Down syndrome: Down's syndrome; Trisomy 21
Down syndrome, a chromosomal disorder, occurs when, instead of the normal complement of two copies of chromosome 21, there is a whole, or sometimes part of an, additional chromosome 21.
A chromosome is a rod-like structure present in the nucleus of all body cells, with the exception of the red blood cells, and which stores genetic information. Normally humans have twenty-three pairs of chromosomes, the unfertilised ova and each sperm carrying a set of twenty-three chromosomes. On fertilisation the chromosomes combine to give a total of forty-six (twenty-three pairs). A normal female has an XX pair and a normal male an XY pair.
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