Fragile X syndrome is the most common genetically inherited form of mental retardation currently known. In addition to intellectual disability, some individuals with Fragile X display common physical traits and characteristic facial features, such as prominent ears. Children with Fragile X often appear normal in infancy but develop typical physical characteristics during their lifetime. Mental impairment may range from mild learning disability and hyperactivity to severe mental retardation and autism. This genetic syndrome is caused by an expansion of repeating segment of FMR-1 gene on X chromosome. Because of scientific advances, improvements in genetic testing, and increased awareness, the number of children diagnosed with Fragile X has increased significantly over the last decade (Stephen and Allan, 2000).
Although the normal function of the FMR-1 gene is not fully understood, it appears to be important early in development. The mechanism by which the normal FMR-1 gene is converted into an altered, or mutant, gene capable of causing disease symptoms involves an increase in the length of the gene.
At the 5' region of the FMR-1 gene, the CGG nucleotide sequence undergoes repeated duplications, forming deoxyribonucleic acid (DNA) repeats that result in a longer gene. The lengthened DNA region is susceptible to a chemical modification process called DNA methylation. When the number of repeats is small (usually from 50-200), as in the case of premutation, the individual often has no signs of the disorder. However, in individuals with a larger number of repeats (more than 200), as in the case of full mutation, the characteristics that are typical of Fragile X are observed (Stephen and Allan, 2000; Frank, 2000).
In families that exhibit Fragile X, both the number of repeats and the length of the chromosome increase with succeeding generations. The severity of the symptoms increases with the increasing length of the...