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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.

What are the causes? View What are the causes?

Medical text written November 1991 by Contact a Family. Approved November 1991 by Professor M Patton, Professor of Medical Genetics, St Georges Hospital Medical School, London, UK and Dr J E Wraith, Consultant Paediatrician, Royal Manchester Children's Hospital, Manchester, UK. Last reviewed August 2004 by Professor Ben Sacks, Down Syndrome Educational Trust, Portsmouth, UK. Additional genetic information provided June 2005 by Dr L Devlin, Senior Registrar in Genetics, Belfast City Hospital, Belfast, UK.

 

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