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Down’s Syndrome: Down's syndrome is caused due to the presence of an additional copy of chromosome 21 (Trisomy of 21). It was the first discovered and described chromosomal disorder in humans. The total number of chromosomes in people affected with Down's syndrome becomes 47. Affected individual has short stature, small, round head, furrowed tongue, partially opened mouth, palm crease, congenital heart disease and mental retardation.