Trisomy syndrome is a genetic condition in which a person has an extra chromosome in their cells. Some synonyms for trisomy syndrome include Down syndrome, Edwards syndrome, and Patau syndrome. Down syndrome is the most common form of trisomy syndrome and is characterized by intellectual disability, distinct facial features, and developmental delays. Edwards syndrome is a less common form of trisomy syndrome, which is characterized by severe intellectual disability, clenched fists, and a small head. Patau syndrome is also a rare form of trisomy syndrome and is characterized by severe intellectual disability, cleft palate, and a small head. Other forms of trisomy syndrome include Turner syndrome and Klinefelter syndrome.