<qa>  What is the most common cause of inherited mental disability?The Fragile X syndrome FXS was first described by Dr. Martin and Dr. Bell in 1943, in families with several patients affected by sex-linked mental disability. This disorder is the most common cause of inherited mental disability. The prevalence of the Fragile X syndrome has been established at 1 in 2,500 males and 1 in 4000 females. 