Basic information for topics, such as “What is it?” and “How many people are affected?” is available in the
Condition Information section. In addition, Frequently Asked Questions (FAQs) that are specific to a certain topic are answered in this section.
Are there specific disorders or conditions associated with Fragile X syndrome?
How is Fragile X syndrome inherited?
How does the
FMR1 gene change as it is passed from parent to child?
Among the other conditions associated with Fragile X syndrome are the following:
The gene for Fragile X is carried on the X chromosome. Because both males (XY) and females (XX) have at least one X chromosome, both can pass on the mutated gene to their children.
The repeats in the promoter part of the
FMR1 gene are unstable, and sometimes the number of repeats increases from one generation to the next.
A premutation gene is less stable than a full mutation gene. So as it passes from parent to child, a premutation gene might expand to become a full mutation gene. The chances of expansion depend on the number of repeats in the promoter of the premutation gene:
FMR1 genes that have 5 to 44 CGG repeats in the promoter are considered normal. When these genes are passed from parent to child, the number of repeats does not increase or decrease.4
FMR1 genes with 45 to 54 CGG repeats in the promoter are considered intermediate, or borderline. An intermediate gene may expand from one generation to the next, depending on which parent has the gene.
About 14% of the time, when a mother passes an intermediate gene to her child, the CGG repeats increase to a number seen with premutations. Research shows that an intermediate gene will not become a full mutation gene in one generation, and so a mother with an intermediate gene will not have a child with a full mutation.
When intermediate genes are transmitted from father to child, they are generally stable and do not increase to premutations.4
Premutation (55 to 199 CGG repeats)
FMR1 genes can expand to a full mutation from one generation to the next. The risk of expansion depends on which parent has the gene and the number of repeats in that gene.
FMR1 gene from the mother with 100 CGG repeats is very likely to expand to a full mutation when passed to the child. An
FMR1 gene from the mother with 70 to 79 CGG repeats has about a 30% chance of expanding to a full mutation in one generation.5
Premutations passed from father to child have almost no chance of expanding to full mutations.5
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