Genetic research has identified many genetic causes of intellectual disabilities (ID, formerly mental retardation). Two leading genetic causes are Down syndrome and Williams syndrome. These and other genetic causes are discussed in a separate section.
During the 1930s, genetic researchers Waardenburg and Bleyer were first to propose a genetic link for Down syndrome. However, it was not until 1959 that research conclusively demonstrated this. Working separately, Jerome Lejeune and Patricia Jacobs demonstrated Down syndrome was caused by an abnormality on chromosome 21. This was called Trisomy 21.
Williams syndrome was first recognized in 1961. However, the genetic cause of the disorder was not discovered until the 1990s. Dr. Colleen Morris and her associates at University of Utah discovered Williams syndrome was caused by a genetic deletion on chromosome 7.
Other medical and genetic conditions that cause intellectual disabilities have been similarly identified. At the University of Florida in 1987, Dr. Charles Williams linked Angelman syndrome to missing portions of chromosome 15. Williams' discovery was further refined in 1997 by Drs. Joseph Wagstaff and Arthur Beaudet. They pinpointed the cause of Angelman syndrome as a mutation, or absence, of a specific gene known as UBE3A. Likewise, in 1943 researchers Martin and Bell were able to link X chromosome problems to Fragile X syndrome. Researcher Herbert Lub provided research confirmation of this in 1969. Learn more about these and other genetic causes of ID here.