Down syndrome is a congenital anomaly that is due to a trisomy (three copies, instead of two) of chromosome 21. Those affected by this syndrome may have intellectual disability and other complications. There are other trisomies that cause major complications in those who suffer, such as Edwards syndrome (trisomy of chromosome 18) and Patau syndrome (trisomy of chromosome 13).
Alterations in the number of sex chromosomes, known as sexual aneuploidies, can occur. Among the most common, Turner Syndrome (X0) and Klinefelter Syndrome (XXY) stand out. Those affected present, among other complications, problems of sterility and in the development of the sexual organs, and, in certain cases, intellectual disability. Down syndrome occurs approximately in 1/800 births.
Edwards Syndrome and Patau Syndrome appear in one of every 5,000 and 16,000 pregnancies, respectively. On the other hand, sexual aneuploidies affect in Turner Syndrome at 1/2500 and in Klinefelter Syndrome at 1/500 – 1/1000 births.