Fragile X syndrome is also known as marker or Martin-Bell syndrome. It is a common cause of intellectual disability, mental retardation, and autism. It is named after Martin and Bell who investigated a family who had mental retardation in multiple male members in 1943. They were able to find the association of the cognitive disorders and unidentified mode of X-linked inheritance. This means that it is inherited in an X-linked dominant pattern.
In 1969, it was discovered that there is excessive genetic material on the X chromosome for males who are affected and for female carriers. Since then, gene mapping for this condition has been improving and the precise defect has been characterized. Due to the improvements and advances in molecular genetics, there is now reliable testing for the condition.
In the United States, it has been estimated (conservatively) that fragile X syndrome affects about 1 in every 2,500 to 4,000 males and 1 in every 7,000 to 8,000 females. The female carrier status has been estimated to be as high as 1 in every 130 to 250 women while male carriers are estimated to be 1 in every 250 to 800 men. It is also estimated that fragile X syndrome affects as many as 10% of cases of undiagnosed mental retardation in males and 3% of undiagnosed mental retardation in females. While the exact frequency is unknown internationally, data collected from both Australia and England were found to be comparable to data from the United States.