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What is Down syndrome Down syndrome is a chromosomal disorder characterized by the presence of an extra 21 chromosome Instead of having 46 chromosomes in each of hisher cells a person with Down syndrome has 47 The extra chromosome may be derived from either the egg or the sperm It is believed that during cell division of a sperm or egg the two 21 chromosomes do not separate properly nondisjunction Thus one cell will have an extra 21 chromosome which later if united with a normal germ cell which has among others one 21 chromosome may lead to trisomy 21 The first cell at conception then has 47 chromosomes instead of normal set of 46 chromosomes When this cell with three 21 chromosomes continues to multiply and the pregnancy is carried to term a baby with Down syndrome will be born This form of chromosome error trisomy 21 is found in about 95 of persons with Down syndrome The 5 percent of babies with Down syndrome who do not have trisomy 21 may have Translocation Mosaicism or partial trisomy In translocation Down syndrome there are also three copies of the 21 chromosome However one of the 21 chromosomes is attached or translocated on another chromosome usually a 14 21or 22 chromosome Three to four percent of children with Down syndrome have translocation About one third to one half of translocations are inherited from one of the parents When this happens the carrier parent has the normal amount of genetic material however one of the 21 chromosomes is attached to another chromosome As a result this individuals total chromosome count is 45 instead of 46 The carrier parent is clinically unaffected because there is no loss or excess of genetic material Doctors refer to a parent like this as a balanced carrier Another form of Down syndrome is known as mosaicism which affects about 1 percent of all people with Down syndrome In mosaicism a faulty
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