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| printer friendly | DOWN SYNDROME | ||||||||||||||||||||
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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. Chromosome abnormalities give rise to specific physical features. The range of cognitive disabilities as well as other attributes is enormously wide in Down syndrome. The majority are in the mild range of cognitive ability. Associated defects may include ear and/or eye defects, an increased propensity for infections and heart defects. A few individuals have the mosaic form of trisomy 21. This means that some body cells have forty-six chromosomes while others have forty-seven. In this form the severity and extent of the condition is dependent upon the proportional relation of normal to abnormal cells. Inheritance patterns However, in a minority of cases (three to four per cent) a mother or father may have a balanced translocation of chromosome 21. In these cases the condition is inheritable. Genetic counselling should be sought in all cases. An ante-natal screening test, the 'Triple Test,' is sometimes used. It involves taking a small sample of blood from the mother, but it is not a definitive test; it is a screening process, the aim of which is to calculate whether the likelihood of having a Down syndrome fetus is sufficiently high to warrant the performance of an amniocentesis. This screening test has a rather high false positive rate, and it is only the amniocentesis which provides a definitive answer. Prenatal diagnosis 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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Contact a Family may also be able to help with information and contacts for conditions not listed - please call the Contact a Family Helpline on 0808 808 3555 Freephone for parents and families (Mon-Fri, 10am-4pm & Mon, 5.30-7.30pm). |
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| This Web Site © Copyright, Contact a Family 2007. Although great care has been taken in the compilation and preparation of this work to ensure accuracy, Contact a Family cannot accept responsibility for any errors or omissions. Any medical information is provided for education/information purposes. It should be noted that most information written about specific disorders includes the most severe scenario. Many individual cases may not display all the features mentioned and the degree of severity of the features that are displayed may vary a great deal between individual cases. You should obtain further information from your medical practitioner. Any links to external websites have been carefully selected but are provided without any endorsement of the content of those sites. Please refer to our detailed Legal page for more information. Contact a Family, 209-211 City Road, London EC1V 1JN Tel: (020) 7608 8700. Registered Charity No. 284912. Company limited by guarantee, registered in England and Wales No. 1633333. HM Revenue & Customs charity tax reference No. XN54769. VAT Reg. No. GB 749 3846 82. |
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