LYCOS RETRIEVER 
Fragile X Syndrome: Chromosomes
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Fragile X is a genetic syndrome carried on the X chromosome. Females have two X chromosomes (one from each parent) and males have one X chromosome (from the mother) and one Y chromosome (from the father). If the X chromosome in males is affected they will have fragile X syndrome. Females will have a second (non-affected) X chromosome, which acts like a back up, and they can be less affected.
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Fragile X exhibits X-linkage. The effect of X-linkage is that the frequency of the syndrome is greater in males than in females. To understand the mechanism of X-linkage some background information on the organization of human chromosomes is needed. Human females typically have two X chromosomes, and human males have one X and one Y chromosome. A female who inherits a chromosome carrying the Fragile X gene from either parent is likely to inherit a normal X chromosome from the other parent. The normal X chromosome could provide the normal gene function and mask the presence of the Fragile X gene in a female.
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Fragile X syndrome, a genetic condition involving changes in the long arm of the X chromosome, is the most common form of inherited mental retardation. Individuals with this condition have developmental delay, variable levels of mental retardation, and behavioral and emotional difficulties. They may ... have characteristic physical traits. Generally, males are affected with moderate mental retardation (since they only have one X chromosome) and females with mild mental retardation.
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Fragile X is caused by an unstable trinucleotide repeat of the series CGG at the FMR1 gene (familial mental retardation) region on the long arm of the X chromosome. Normal unaffected individuals have between 5 to 50 ( average is about 29) CGG triplet repeats in the FMR1 promoter region. In unaffected but pre-mutated patients, there were found to be up to 200 repeats. Affected patients have >200 to thousands of CGG trinucleotide repeats. The number of CGG repeats, along with the degree of methylation of this gene determine the phenotype of the affected individual. The Fragile X phenotype results from a loss of function of a protein, and not from the production of an abnormal protein in this gene region.
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In order to understand fragile X syndrome it is important to understand how human genes and chromosomes influence this condition. Normally, each cell in the body contains 46 (23 pairs of) chromosomes. These chromosomes consist of genetic material (DNA) needed for the production of proteins, which lead to growth, development, and physical/intellectual characteristics. The first 22 pairs of chromosomes are the same in males and females. The remaining two chromosomes are called the sex chromosomes (X and Y). The sex chromosomes determine whether a person is male or female.
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Fragile X tends to be milder and less obvious in females. A female normally inherits two X chromosomes, one from each parent. If one of the X chromosomes has the fragile X mutation, the other X chromosome, inherited from her unaffected parent, can partially compensate for the incorrect genetic information. About half of affected females show no symptoms of fragile X syndrome. The other half has symptoms, especially learning disabilities, behavioral problems or intellectual impairment.
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