Long ago in the 19th century, a British doctor named Langdon Down came across an abnormality that was observed to have severe effects on the developmental milestones and cognitive potential of an individual. This disorder was termed after its discoverer as Down syndrome in the year 1866. However, at that point of time he could not comprehend the genetic alteration associated with the disorder. It was only in 1959 that the genetic configuration of Down syndrome was finally brought to light.
A healthy fetus inherits twenty-three pairs of chromosomes each from the mother and father. Each of these chromosomes is a carrier of genes that are cardinal for the normal functioning of all body systems. Research linked to Down syndrome projects the presence of an additional 21 chromosome in affected individuals. During the occurrence of this trisomy 21, the fetus as a random error gets 2 copies of chromosome 21 from the mother and 1 copy of the same chromosome from the father, thereby resulting in the presence of three chromosomes 21.
In 95 percent of cases with Down syndrome, the extra chromosome 21 is inherited from the mother. However, when this is not the case then the condition is slightly deferred in the form of translocation Down syndrome and mosaic Down syndrome. During translocation, only some of the extra chromosome 21 genes that are associated to chromosome 14 are inherited during conception itself. The reasons for such translocations are either random errors or one of the parents being a balanced carrier of a translocation. In cases of mosaic Down syndrome, all the cells in the body do not inherit the extra genes from the 21st chromosome. Therefore, the effect of the abnormality is only partial. Studies related to Down syndrome have till now been able identify 400 genes of the 21st chromosome. However, the exact correlation between the additional genes on chromosome 21 and the cause of the abnormality still need to be identified.
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