Sunday, 27 May 2012

Down Syndome


Down syndrome is the result of a  by a chromosomal defect caused by an extra copy of chromosome 21 and affects “1 in 740 newborns” (2008). It is most often associated with “intellectual disability, a characteristic facial appearance and poor muscle tone (hypotonia) in infancy(2008)”. People with down syndrome are often born with heart defects and have an increased risk of developing numerous medical conditions. Medical conditions include gastroesophageal reflux, celiac disease, under-active thyroid gland, hearing and vision problems, leukemia and Alzheimer disease (2008). Adults with down syndrome tend to develop Alzheimer's by the age of 50 (2008). Unfortunately most people with down syndrome have a much shorter life span than normal (Campbell et al., 2009).


“Humans normally have 46 chromosomes in each cell(2008)” 23 chromosomes are inherited from the mother and 23 from the father which are then paired to form 23 pairs per cell. Chromosome 21 has about 47 million base pairs  and represents about 1.5% of the total DNA in a cell making it the smallest human chromosome(2008). Chromosome 21 is the second human chromosome to be sequenced, it was sequenced in 2000 by researchers working on the 'Human Genome Project'(2008). The estimated number of genes on chromosome 21 is somewhere between 300 and 400 genes (2008).

Down syndrome is most commonly caused be trisomy 21. This means that every cell has three copies of chromosome 21 instead of two (Mosaic Down syndrome)(2008).  It can also occur when only part of chromosome 21 becomes “attached to another chromosome during the formation of reproductive cells or early in fetal development(2008)”. In this case it means the individual will have two copies of chromosome 21 as well as extra bits of chromosome 21 attached to a different chromosome (Translocation Down syndrome)(2008).


Down syndrome, in most cases, is not inherited. Mosaic down syndrome caused by trisomy 21 is the result of a random event that occurs during cell division in the early stages of fetal development (2008).  Therefore Mosaic down syndrome can not be inherited. Translocation down syndrome on the other hand can be inherited. Even if someone is unaffected, they can still carry a “rearrangement of genetic material between chromosome 21 and another chromosome(2008)”. This type of rearrangement is known as 'balanced translocation' because no extra material from chromosome 21 is present (2008). People who carry this trait have an increased risk of having a child with down syndrome (2008).


Down syndrome is not the only disorder associated with chromosome 21(2008). Acute lymphoblastic leukemia and acute myeloid leukemia are just two forms of cancer that can result from rearrangements of genetic material between chromosome 21 and other chromosomes(2008). There is now a test that can be done to determine whether a baby will be born with down syndrome (2008). To find out more visit this website:  http://ghr.nlm.nih.gov/chromosome/21


 


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 Image sourced from 'Google Images'


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