The syndrome of physical and mental symptoms that present a clinical disease or disorder is hereditary. This phenomenon is due to an irregularity in the chromosome during cell division occurs in the embryonic stages. Those who have Down syndrome in their body cells have 46 chromosomes instead of 47 chromosomes. Extra chromosome in the body of a person who has Down syndrome, causes physical and mental changes. As far as the most worrying factor in Down syndrome, is mental retardation. Down syndrome is the subject of medical issues that have been discussed, but rarely in legal writing on legal issues of the people, is the answer found. Therefore it is necessary that the law be examined. In this article we try, following the introduction of a variety of mental retardation and Down syndrome to describe and compare them with those incapable of law. And their position in the jurisprudence of the evidence in this case Not mature finally prove that they are in financial and non-financial and madness when they should be.