LYCOS RETRIEVER 
Fragile X Syndrome: Chromosomes
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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 a defect on the 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.
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Fragile X Syndrome affects males and females differently. Males, because they have only a single X chromosome, usually suffer more severe effects of Fragile X Syndrome and are twice as likely to be affected by the mutated gene as females. These include intellectual deficits (ranging from mild to severe); behavior characteristics such as hand flapping and biting, ADD, speech problems, autistic behavior, and aversion to touch and sound; and physical features, including long faces and prominent ears, weakened connective tissue, and macroorchidism (enlarged testicles). Some of these problems don’t show up until puberty. Males with the disease are more likely than not to have mental retardation. Nevertheless, a significant minority—around 20%—of males with the Fragile X mutation exhibit no symptoms, or very mild symptoms.
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The mutation in the FMR1 (Fragile X Mental Retardation Protein) gene causes Fragile X Syndrome. The CGG is repeated more than 200 times. This abnormally stretched CGG repeat stops the FMR1 gene from working. The FMR1 gene produces a protein. The loss of protein leads to signs and symptoms of Fragile X Syndrome. The FMR1 gene is on the X chromosome.
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Commonly referred to as the Martin-Bell syndrome or the Marker X syndrome, Fragile X is the most common form of inherited mental retardation in males and a significant cause in females. The inheritance is different from common dominant or recessive inheritance patterns. A fragile area on the X chromosome tends to repeat bits of the genetic code. The more repeats, the more likely there is to be a problem. Boys and girls can both be affected, but because boys have only one X chromosome, a single fragile X is more likely to affect them.
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The characteristic problems of fragile X syndrome are caused by the full mutation of a specific gene called the fragile X mental retardation 1 gene (FMR1). Genes are the building blocks of chromosomes. Each person has 23 pairs of chromosomes. One pair, called the sex or X and Y chromosomes, determine whether a person is male or female. Females have two X chromosomes and males have one X and one Y chromosome. In fragile X syndrome the gene mutation is on an X chromosome.
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Since males have only one X chromosome (where the FMR1 gene is located), all boys with this defect show characteristics of Fragile X Syndrome. Females, on the other hand, have two X chromosomes (each with the FMR1 gene). Therefore, girls with this defect in one FMR1 gene may still be able to use the other active gene to produce some of the normal protein. As a result, girls may not be as severely affected as boys.
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