Fragile X syndrome


How is fragile X syndrome inherited?

The Fragile X mental retardation 1 (FMR1) gene is found on the X chromosome, meaning fragile X syndrome is an X-linked disorder and is passed from one generation to the next on the X chromosome. Typical men have one X chromosome and one Y chromosome and typical women have two X chromosomes. When a child is conceived, that child inherits one of their mother's X chromosomes, and either another X chromosome from their father (resulting in a female child, XX) or the Y chromosome from father (resulting in a male child, XY). Individuals who have 55 to 200 CGG trinucleotide repeats are said to have a premutation and are called premutation carriers. Men who have the premutation will pass their relatively stable repeat size to all of their daughters because they pass their affected X chromosome to their daughters. Any sons of men who have the premutation can NOT inherit the premutation since sons inherit only a Y chromosome from their father. Women who have the premutation can pass any of the following to their sons and daughters who receive the affected X chromosome:

  • A premutation of relatively same size to their own
  • A premutation of larger size to their own
  • A full mutation of greater that 200 repeats, resulting in fragile X syndrome

Women who have the full mutation (greater than 200 CGG repeats) will pass along the full mutation to any of her children who receive the affected X chromosome. Men with the full mutation (who have fragile X syndrome) typically do not have children of their own. For more information about how fragile x syndrome is passed down in families and to have your family assessed, please visit The National Society of Genetic Counselors and click the link "Find a Genetic Counselor".

Sherman, S. L., Morton, N. E., Jacobs, P. A. and Turner, G. (1984), The marker (X) syndrome: a cytogenetic and genetic analysis. Annals of Human Genetics, 48: 21–37. doi: 10.1111/j.1469-1809.1984.tb00830.x

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