Fragile X syndrome

Fragile X syndrome is a recently recognized disorder due to a defective gene on the X chromosome. It is so named because a small portion of the tip of the X chromosome seems susceptible to breakage. It is the leading cause of mental retardation among newborns.

Fragile X syndrome affects males more than females and results in learning difficulties, mental retardation, and physical abnormalities such as over sized ears, elongated forehead, enlarged testes, and double jointedness. 

The syndrome may also be involved in autism in which the individual exhibits extreme withdrawal and refusal to communicate.

Fragile X syndrome can be diagnosed by amniocentesis.

Although the symptom is largely sex-linked, 20 to 50 % of males who inherit the trait are unaffected, but they can pass it on to their daughters. although the daughters also are unaffected, their children, both male and female, may suffer mental retardation.

Researchers have suggested that this pattern of inheritance could be explained by maternal imprinting of the gene.

On this theory, the father's fragile X gene becomes chemically changed as it passes through the daughter so that the gene is expressed in the daughter's offspring.