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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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