The fragileX chromosome is associated with a characteristic constriction fragile site on the long arm Visuals Unlimited

Expanding trinucleotide repeats In 1991, an entirely novel type of mutation was discovered. This mutation occurs in a gene called FMR-1 and causes fragile-X syndrome, the most common hereditary cause of mental retardation. The disorder is so named because, in specially treated cells of persons having the condition, the tip of the X chromosome is attached only by a slender thread (< Figure 17.5). The FMR-1 gene contains a number of adjacent copies of the trinucleotide CGG. The normal FMR-1 allele (not containing the mutation) has 60 or fewer copies of this trinucleotide but, in persons with fragile-X syndrome, the allele may har

Table 17.1 Examples of genetic diseases caused by expanding

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