Fragile X syndrome is a genetic condition that causes intellectual and behavioral changes. This condition is the most diagnosed genetic cause for intellectual disability; second only to Downs Syndrome.

fragile x syndrome

In the 1970s, the genetic abnormalities associated with the fragility of a segment of the X chromosome were identified, which led to the discovery of the condition called fragile X syndrome. But it was not until the 1990s that the cause of this fragility was discovered a mutation in the FMR1 gene. Once this gene is altered, it no longer passes instructions for the production of a protein essential for the nervous system.

The lack of this protein leads to a significant change in intellectual ability and behavior. These symptoms, however, are quite common in many other conditions, which is why the diagnosis of fragile X syndrome does not always occur quickly.

Current studies show that the disease affects one in 4,000 boys and one in 6,000 girls. The lower frequency in females is due to the fact that the females have two X chromosomes, which protect her from developing fragile X syndrome.

Fragile X syndrome

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Causes and symptoms of fragile X syndrome

With the exception of muscle flaccidity, which may be present at birth, signs only appear over time. Fragile X syndrome results from a mutation in a certain region of the FMR1 gene, which is located on the X chromosome. This change is very peculiar and is characterized by an increase in the number of CGG repeats, which are components of the cytosine guanine of DNA.

Fragile X syndrome

Such an increase causes the gene to be silenced and the resulting protein to be encoded. In normal development, there are 6 to 50 copies of the CGG crack in that part of the FMR1 gene. In fragile X syndrome, there are more than 200 repetitions of the same sequence. There is, however, an intermediate condition that involves pre-mutation, which comprises individuals who have 50 to 200 CGG copies. In this range, people usually do not manifest cognitive or behavioral problems. But, they can transmit the syndrome to their descendants, causing the complete expression of the disease, with more marked symptoms in males and reduced syndromes in females.

Tests and diagnostics

Because the symptoms resemble those of other disorders involving intellectual development, the diagnosis of fragile X syndrome requires a molecular test – DNA analysis – to confirm. Using a blood sample, the test can detect both mutation and pre-mutation in the FMR1 gene by determining the number of copies of the CGG sequence in this part of DNA.

Treatments and preventions

There are no ways to prevent X chromosome failure and the resulting mutation of the FMR1 gene. However, because it is a disease with a peculiar pattern of inheritance, it is possible to identify relatively easily those carriers of the pre-mutation, who may have descendants with the disease.

Thus, especially for couples with a family history of cognitive and/or behavior disorders involving delayed intellectual development, it may be beneficial to investigate this condition and, if necessary, to receive genetic counseling.


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External Resources

Genetics Home Reference – NIH

Support program: Autism Sibling Support Initiative

Support program: Sibling Support Project

Maryland Developmental Disabilities Council

Maryland Early Intervention and Special Education Services

Pathfinders for Autism

Disability Scoop

Maryland Preschool Special Education

Maryland Department of Education Division of Early Childhood