- Fragile X syndrome is characterised by particular physical features, varying degrees of learning difficulties and behavioural and emotional problems and affects about 1 in 4,000 males and between 1 in 5,000 and 1 in 8,000 females
- The condition is due to a change in the information in the FMR-1 gene that impacts on the production of an important protein in the brain called the FMR-1 protein
- Women have two copies of the X chromosome (XX); men have only one X chromosome and a Y (XY); there is no FMR-1 gene copy on the Y chromosome
- The change in the information in the FMR-1 gene is to the number of repeats in a sequence of one of the ‘code words’. When the number of repeated code words increases over a critical number, the gene becomes faulty. The length of the repeat sequence can be described as short, medium and long. In most people, the repeat length size is short.
- Individuals with the medium repeat size have a working copy of the gene but are premutation carriers. They do not have fragile X syndrome, so are not affected intellectually but are at risk for developing a neurological condition after about 50 years of age; women are also at risk for early menopause
- The long repeat size is a full mutation and its presence makes the gene faulty so that it can no longer do its normal job in the cell
- Men with a full mutation will have fragile X syndrome
- Women with the full mutation in one of their FMR-1 gene copies and a working copy on the other partner X chromosome, will be carriers of the faulty gene (a genetic carrier for fragile X syndrome). They may be mildly affected with fragile X syndrome depending on how many of their cells are expressing the faulty FMR-1 gene copy
- The pattern of inheritance in families of the faulty gene causing fragile X syndrome is described as X-linked recessive inheritance but is more complex than the usual pattern of inheritance of X-linked genes. The size of the repeated sequence can increase when inherited from the mother: a mother with a medium size repeat (premutation) can have children with a long repeat size in their FMR-1 gene copies (full mutation)
- Where the mother is a carrier of the faulty FMR-1 gene and the father has only a working copy of the gene, in every pregnancy, their sons have 1 chance in 2, or a 50% chance, of inheriting the faulty gene and having the condition. Their daughters have 1 chance in 2, or a 50% chance, of inheriting the faulty gene copy and being a usually less severely affected genetic carrier for fragile X syndrome
- Genetic testing can determine if an individual is a carrier of a medium or long FMR-1 gene change where there is a family history of fragile X syndrome or a blood relative with the gene change
- Where one of the partners in a couple is a carrier of a changed FMR-1 gene, the genetics team can provide information about the condition and discuss their risk for having an affected child with fragile X syndrome and their reproductive options
(source : Fragile X Syndrome Fact Sheet by Centre for Genetics Education).
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