Down's syndrome is due to an extra chromosome, which combines to the chromosome pair $21$. Down syndrome is caused due to trisomy, in which the fusion of a normal $n$ gamete (haploid) takes place with an $n+1$ gamete (diploid). J. Langdon Down first described the developmental defect produced by trisomy in $1866$ . For this reason it is called Down's syndrome. Persons having Down's syndrome generally have a short, stocky build short hands, flattened facial features and poor muscle tone. In addition they are mentally retarded.