Description of Down syndrome
Down’s syndrome is a genetic disorder that is characterized by mental retardation mostly caused by trisomy 21 (three sets of chromosomes instead of a pair) which is present in 95% of persons with Down syndrome. A combination of normal diploid and trisomy 21 cell known as mosaicism occurs in 2% while 3% have a Robertsonian translocation whereby all or part of an extra chromosome 21 is joined to another chromosome. Majority of the chromosome-21 translocations occur sporadically but some are inherited from a parent with the parent with the defective chromosome. The occurrence of trisomy 21 is due to a meiotic error in which disjunction from the primary oocyte does not occur. The incidence of fetal trisomies is directly linked to maternal age. The chances of having a child with Down syndrome is 1/1,300 for a 25-year-old woman; at age 35, the chances are increased to 1/36 but at age 45, the chances of a having a child with Down syndrome increases greatly to 1/30. Maternal age can therefore be regarded as a primary screening test but amniocentesis and maternal serum screening are carried out to determine if the foetus is affected by Down syndrome[ CITATION LAK03 \l 2057 ].
Down syndrome is mainly associated with from mild to severe mental retardation or cognitively disability. Children with attending school who have Down syndrome experience language difficulties and their communication and problem solving abilities are below those of their normal counterparts of the same age. As for the adults with Down syndrome, they have a high prevalence of early Alzheimer's disease and impairment of their cognitive abilities. A number of congenital malformations and acquired diseases occur with increased frequency in persons with Down syndrome such as congenital heart defects, ophthalmic disorders, and loss of hearing ability, leukemia and hypothyroidism. The leading causes of mortality, especially during early childhood are congenital heart disease and pneumonia[ CITATION Guy06 \l 2057 ]. Down syndrome is also associated with certain developmental delays and physical features such as flattened nose bridge, decreased muscle tone, one deep crease on the hand palm instead of two, upward slant of the eyes, small chin (microgenia), abnormally round face, shorter limbs among others. It is important to note that these physical characteristics don’t occur in all people with Down syndrome neither are they unique to the disease but are often present in many other genetic and chromosomal disorders[ CITATION CJE99 \l 2057 ].
Often (95%) Down syndrome is due to inheriting an extra chromosome from the mother. It can also be due to inheriting extra genes of chromosome 21 (3-4% incidences) which may be random or one parent may carry a balanced translocation. In the condition called mosaic Down syndrome extra genes in chromosome 21 are inherited but not in all cells[ CITATION Siend \l 2057 ].
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