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                               Down syndrome ( also known as trisomy 21) is a genetic disorder in which a person is born with an extra copy of their 21st chromosome. This disorder causes the patient to go through physical development delays, mental disabilities and a variety of distinguishable facial features. The possibility of being affected by down syndrome occurs by chance, the parents of the infected persons are genetically normal. This disability is permanent, and it usually shortens life expectancy. Although the disease has no known cure, people infected with down syndrome can still live very healthy and fulfilling lives. Medical advances have created support for people and their families that have been affected by down syndrome, providing opportunities to help prevail over the challenges of this disorder.                                   In a normal cell, there are 23 pairs of chromosomes, making it a total of 46. 23 of the chromosomes come from the mother’s egg and the other 23 comes from the father’s sperm. The XY chromosome includes the Y chromosome found in the sperm and the X chromosome found in the egg. In Down syndrome, there are three copies of chromosome 21 instead of two, this is called a chromosomal abnormality. The additional chromosome is either as a whole (trisomy 21) or parts of it (translocations). The effects of Down syndrome are different from person to person depending on how long the extra copy, the infected genetic background, and random possibilities. Down syndrome can affect all humans and similar effects have been found in other species (e.g chimpanzees and mice).  Scientists have created transgenic mice with the human chromosome 21 and their own chromosomes.                        There are three chromosomal patterns that result in Down syndrome: trisomy 21, translocation and mosaicism.  Trisomy 21 also known as nondisjunction is when a baby has 21 chromosomes instead of two. When a pair of 21 chromosomes fails to separate properly either in the egg or sperm. The extra chromosomes go on to be replicated into the cell in everybody. 95% of people with Down syndrome have trisomy 21.                         Translocation occurs in only 3% to 4% of all cases. In translocation, a part of chromosome 21 breaks off during cell division then attaches itself to another chromosome. The extra piece of the 21st chromosome causes the characteristics of down syndrome, translocation and may show that the infected person’s parents are carrying chromosomal material that is arranged in an unusual manner, genetic counselling can be sought to ascertain more information when these circumstances occur.                       Mosaicism is when disjunction of chromosome 21 takes place in one of the cell divisions after fertilization. After this happens there is a mixture of two types of cells, some contain 46 chromosomes and others 47. The cell that contains 47 chromosomes has an extra 21st chromosome. This type of Down syndrome only occurs in 1% to 2% of all cases of Down syndrome. There is a slight difference between trisomy 21 and Down syndrome. Nondisjunction is when the homologous chromosomes don’t separate properly during cell division. There are three types of nondisjunction, when the sister chromatids don’t separate during mitosis, when the sister chromatids don’t separate during meiosis II and when the homologous chromosomes don’t separate during meiosis I. Nondisjunction causes the daughter cells to have abnormal chromosome numbers. While trisomy 21 is when there is an extra copy of the chromosome.                   The average age that people with down syndrome live is increasing from 10 years in 2010 to 60 years now. However their survivorship is determined by some heart problems by some factors; about 12% die in their first year, 60% of those with cognitive heart problems live to 10 years old and 50% live to 30 years old. 85% of those with no heart problems live to 10 years old and 80% live to 30 years old. Only 10% lives to 70 years of age.                      Down syndrome cannot be cured. During pregnancy, if the pregnant woman goes through prenatal scanning and the baby is found to have down syndrome 95% of those pregnancies are terminated. 20% to 50% of individuals with Down syndrome have problems of the thyroid gland, low thyroid is the most common form, occurring in half of all infected persons. These problems can be due to a poorly or totally non-functioning thyroid at birth also known as congenital hypothyroidism which happens very rarely or develops later due to an attack on the thyroid by the immune system. The rate of heart diseases in newborns with down syndrome is around 40 %, out of all the diseases only about 80%have a ventricular septal defect or atrioventricular septal defect (this one is more common). The mitral valve problems become common as people grow even in the people without heart problems at birth. People with Down syndrome have a lower risk of their arteries being hardened. Other problems that may include the arteries are tetralogy of Fallot and patent ductus arteriosus.                    The risk of having cancer in infected persons is not changed but the risk of leukaemia and testicular cancer is increased and risk of solid cancers is reduced. Solid cancers are less common because of the increased expression of tumour suppressor genes that are in the chromosome 21.Cancers of the blood are 10 times more common in children than with down syndrome. Specifically, acute lymphoblastic leukaemia is about 20 times more common and the megakaryoblastic form of acute lymphoblastic leukaemia is 500 times more popular. Transient myeloproliferative disease is a disorder of blood cell production that does not happen outside of Down syndrome, it affects about 7% of newly born babies. The disorder is not serious but can be sometimes can be deadly. Most times it is resolved without treatment but, those that have it are at risk of developing acute lymphoblastic leukaemia.                    The  hypothalamic dysfunction is the primary cause of GH deficiency and growth retardation, although there has been minor research on how down syndrome affects our systems (excretory, nervous) there hasn’t been any major research with trustworthy results. People with Down’s syndrome usually tend to develop hypothyroidism but sometimes they can have hyperthyroidism although this is much less common. Hypothyroidism is hardly ever present at birth and it is more likely for people with down syndrome to develop it as they get older. This usually happens because of autoimmunity meaning when the body’s defence system attacks itself it is sometimes called hashimoto’s hypothyroidism (although there can be other causes).  Trisomy  pregnancies occur when the embryo has three copies of a chromosome rather than the normal two. The most common trisomy is Down’s syndrome or trisomy 21. 

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