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The formal story began in 1866 when a physician named John Langdon Down published an essay in England in which he described a set of children with common features who were distinct from other children with mental retardation Down was superintendent of an asylum for children with mental retardation in Surrey England when he made the first distinction between children who were cretins later to be found to have hypothyroidism and what he referred to as Mongoloids In the 1970s an American revision of scientific terms changed it simply to Down syndrome Chromosomes are thread-like structures composed of DNA and other proteins A person with down syndrome are short in stature and have a small round head with a high flattened forehead and fissured dry lips and tongue They are present in every cell of the body and carry the genetic information needed for that cell to develop Genes which are units of information are encoded in the DNA Human cells normally have 46 chromosomes which can be arranged in 23 pairs Of these 23 22 are alike in males and females these are called the autosomes The 23rd pair are the sex chromosomes X and Y Each member of a pair of chromosomes carries the same information in that the same genes are in the same spots on the chromosome One cell has two 21st chromosomes instead of one so the resulting fertilized egg has three 21st chromosomes Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation In this case two breaks occur in separate chrom osomes usually the 14th and 21st chromosomes There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosome So while the number of chromosomes remain normal there is a triplication of the 21st chromosome material There is a wide range of mental retardation and developmental delay noted among children with Down syndrome Some
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