The Down syndrome, also known by the name of trisomy 21, was described for the first time in 1866 by English physician John Langdon Down. Le Jeune to identify, in 1959, the cause of its occurrence in an abnormality of chromosome 21.
More precisely, there are three types of chromosomal abnormalities (all still on chromosome 21) which result in final Down syndrome: in 95% of cases are present in every cell in the body three chromosomes 21 (one more than normal), a rather small number of individuals instead presents a mosaic form of the syndrome, for which only some cells have three copies of that chromosome, finally, just rarely, trisomy 21 is caused by a chromosomal abnormality called Robertsonian translocation in which a fragment of chromosome 21 is joined to another chromosome, usually on 14 or 22. Individuals are healthy carriers of this anomaly but have a high probability of generating a child with Down Syndrome (speaking here of Down Syndrome family).

People with this syndrome have characteristic features such as round face, short nose, eyes cutting eastern narrow nasal bridge, epicanthal folds (skin-fold the inner corners of the eye), small ears, fingers and neck, stubby, shorter than the average. The syndrome includes medical complications including heart disease, increased susceptibility to infection due to a partial deficiency of the immune system, susceptibility to leukemia, low vision and hearing impairment, spinal anomalies, hypotonia of muscles. There are mental retardation and language problems.