A genetic syndrome due to a faulty gene on the X chromosome
Fragile X syndrome is a genetic disorder involving the X chromosome. The X chromosome is a sex chromosome. Males normally have only one X chromosome (the other is Y chromosome) whereas females two X chromosomes. Fragile X syndrome is caused by a genetic defect, particularly in the fragile X mental retardation 1 (FMR1) gene that is located on the X chromosome. The expansion of the CGG triplet repeat within the FMR1 gene may cause fragile X syndrome. Depending on the number of CGG repeats, the allele may be described as normal (with no apparent symptoms), permutation (higher risk), or full mutation (showing symptoms). The condition is often associated with mental retardation and physical abnormalities. In males, this condition may cause behavioral features such as ADD, ADHD, and autism. Most of the males affected with this condition have intellectual or learning disabilities. Physically, they tend to have large ears, long ears, and macroorchidism (in post-pubertal males). In females, there may also be intellectual and learning disabilities. However, the intellectual disability is usually milder than in males.
The genetic defect on FMR1 gene may prevent the production of the protein, fragile X mental retardation 1 protein (FMRP). FMRP is essential in the normal neural development and in the proper functioning of the nervous system. The lack or insufficiency of this protein leads to symptoms of fragile X syndrome.
Abbreviation / Acronym: FXS
- Martin-Bell syndrome
- marker X syndrome