Razi Journal of Medical Sciences

 ABSTRACT

 The fragile X syndrome is the most frequent cause of inherited mental retardation. The fragile site is on the long arm of X chromosome in X q27.3 region. Incidence of syndrome is 1 in 2000 in males and 1 in 2500 in females. This fragile site is visible only with using of special cultural technices. Since females have two X chromosomes, this signs apear less than males. The females who carry this gene ordinary don’t have mental defeciency but they can transmit this gene to their children about 50%. This syndrome is a disease dependent to X chromosomes and the people who full mutation have some problemes to learn, severe mental retardation and autism. 33% of female carriers have mental retardation. This disorder is caused by an expansion of a DNA triplet repeat in the upstream region of the FMR1 gene. In health people with normal (CGG)n or (cytozine, Goanine, Goanine), represented as (CGG)n with n= 6 to 40 in normal chromosomes. Fragile X premutation carriers have between 52 to 200 copies of repeats, affected individuals have greater than 200 copies (full mutation) of repeat. Mutation other than the CGG expansion are known but are considered very rare. The carriers who have this gene aren’t known in normal situation. Cytogenetic analysis in affected person with full mutation shows a fragile site in Xq27.3 in 10-40% of cells.