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Fragile X)
Fragile X Syndrome is the most common cause of mental retardation, affecting approximately 1 in 1500 males and 1 in 3000 females. It is a genetic disorder caused by a mutation of the FMR1 gene on the X chromosome. Typically the FMR1 gene contains between 6 and 53 repeats of the CGG codon. In people with the disorder, the FMR1 allele has over 230 repeats. Expansion to such a degree results in a methylation of that portion of the DNA, effectively silencing the expression of the FRM1 protein. The characteristic constriction of the X chromosome at the chromosomal locus Xq27.3 is caused by this methylation.
Because men have only one copy of the X chromosome, males with sufficient trinucleotide expansion are symptomatic, while females have two X chromosomes and thus double the chance of a working allele (unless parents are related). Aside from mental retardation, the most obvious indicators include physical differences and behaviors commonly associated with autism. Of the former, the most readily visible are an elongated face and large or protruding ears, but others are also frequently present. Of the latter, behavioral stereotypy and atypical social development are the most frequently observed.
Because the underlying cause is genetic, there is no cure. The syndrome can be treated through behavioral therapy, special education, and when necessary, treatment of physical abnormalities. Persons with Fragile X in their family histories are advised to seek genetic counseling to assess the likelihood of having children who are affected, and how severe any impairments may be in affected descendants.
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de:Fragiles X-Syndrom
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