Chromosome trisomy 21 is a type of aneuploidy (a genetic disorder) which causes Down syndrome (DS). This disease has specific symptoms such as mental retardation, slower mental and physical development and distinctive facial and head features. The life expectancy of mongoloid children is in average one half shorter than at normal life expectancy. The age of the mother at conception has an effect on the frequency of Down syndrome, therefore, an older mother is more likely than a younger mother to have a baby with the syndrome. The disease can be detected at the prenatal stage with the help of different procedures such as biopsy and amniocentesis. Some new and less risky procedures were recently developed. There are also several tests for prenatal detection of the disease which are not so reliable. The disease and its causes are being studied but the number of information is still too small to expect finding the cure or an appropriate method of treatment in the next ten years or even longer. New possibilities for further research and understanding of the disorder have been created with the emersion of mouse models with DS.