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How Does the Premutation Differ from the Full Mutation

The major factor that determines the presence or absence of Fragile X Syndrome is the number of cytosine, guanine, guanine nucleotide (CGG) repeats in the Fragile X mental retardation gene (FMR-1) on the X chromosome. Typically, if the number of repeats is in excess of 200, it triggers the methylation of the CpG island, a regulatory region for the FMR-1 gene. As a result, the production of the Fragile X mental retardation protein (FMRP) is shut off and the absence of FMRP results in Fragile X Syndrome.

This article includes:

  • A brief discussion of premutation and mutation
  • A table that outlines the differences between premutation and mutation

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Genetic Counseling: Issues and Information

A genetic counselor is a professional who works with a family and the physician to obtain a detailed family history and explain the genetic inheritance pattern, reproductive options and genetic testing related to the diagnosis of Fragile X. The genetic counselor provides family members with professional and emotional support as needed. Because of the role the genetic counselor plays with the physician in giving and receiving information, it is important for family members to meet with a genetic counselor.

This article discusses:

  • The genetic counseling process
  • A genetic overview of Fragile X Syndrome

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Mosaicism in the Fragile X Syndrome

Among males who have the Fragile X mutation, a group of individuals, termed “mosaic,” has different sizes of the CGG repeat expansion in different cells. In other words, some of the cells in mosaic males carry a fully expanded CGG repeat with more than 200 copies while other cells carry a partially expanded repeat described as a “premutation” with 60 to 200 copies. These individuals, termed “size” mosaics, represent the most common form of mosaic males. In addition, there are occasional males who are “methylation” mosaics, carrying the full mutation in both a methylated and an unmethylated state. Mosaic females also have been observed, but, for unknown reasons, with much less frequency. As a consequence, this article focuses only on mosaic males.

This article includes:

  • The origin of mosaicism in Fragile X
  • The identification of mosaicism
  • Developmental delay in mosaic males

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Last Update
October 28, 2014
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Last Update
October 28, 2014
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