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Down syndrome is also known as trisomy 21, trisomy G and mongolism. The first description of a child who presumably had Down syndrome was provided by Esquirol in 1838. Eight years later Seguin described a patient with features suggestive of an anomaly, which later became known as Down syndrome. In 1866 John L. Down published a paper accurately describing some of the characteristics of this syndrome which today bears his name. In 1959, Lejeune and Jacobs independently determined that Down syndrome was caused by Trisomy 21. In 1974 Nebuhr suggested that the "Down syndrome phenotype" might be caused by the duplication of only a part of chromosome 21 band q22, which itself, represents about one half of the long arm. Researches have continued to unravel the genetic basis of Down syndrome. Down syndrome is an easily recognized congenital, autosomal (non-sex chromosomes) anomaly characterized by generalized growth deficiency and mental deficiency affecting 1 in 600 to 1 in 1000 live births. Approximately 95% of Down syndrome cases have extra chromosome 21, making the chromosome count 47 instead of the normal 46. The other 5% are accounted for by other chromosomal abnormalities including translocation (3%) and mosaicism (2%) or partial trisomy. In translocation cases the extra chromosome is not free, but is translocated on another chromosome, usually on chromosome 13 or 15 (D) groups, therefore the count remains 46. In mosaicism, some cells have 46 chromosomes and other cells have 47. These anomalies are mild with intelligence approaching normal. There are three types of mosaicism; cellular mosaicism, tissue mosaicism and chimerism. Maternal age plays an important role in the frequency of Down syndrome. With increased maternal age, the incidence of occurrence increases as well. There appears to be no racial, social, economic or gender predilection.
  Despite the evidence for variation of many characteristics, the literature on Down syndrome has exaggerated the...

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