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Fragile X syndrome, also known as Martin-Bell syndrome, is a genetic syndrome and the most common form of inherited mental retardation, especially in males. Impairments caused by fragile X syndrome are ranging from learning disabilities to more serious cognitive and intellectual disabilities. Fragile X syndrome most commonly manifests as autism or “autistic-like” behaviors, accompanied with different physical and behavioral symptoms and delay in speech and language development.


Fragile X syndrome results from damage in the FMR1 gene. This gene is required for synthesizing a protein essential for the proper functioning of the brain. The gene is needed for learning and memory, and also seems to be involved in development of axons, formation of synapses, and the wiring and development of neural circuits. If there is a mutation in FMR1 gene, the body produces too little of the protein, or none at all. Normally, the FMR1 gene contains between 6-55 repeats of the CGG code. In people with the fragile X syndrome, the FMR1 allele has over 230-4000 repeats of this code. The more the gene’s code repeats, the more likely there is to be a problem.

The FMR1 gene is located on the X chromosome and this is why the disease is more common in males. Mutation at FMR1 gene occurs in 1 out of about every 2000 males and 1 out of about every 259 females.Symptoms

Individuals with fragile X syndrome have varying degrees of mental retardation or learning disabilities and behavioral and emotional problems. Males tend to be more severely affected than females. Along with the intellectual disability, major symptoms of the syndrome include an elongated face, large or protruding ears, flat feet, larger testes (macroorchidism), and low muscle tone. Speech problems may manifest as cluttered speech or nervous speech. Behavioral characteristics usually involve stereotypic movements, atypical social development, shyness, limited eye contact, memory problems etc.

Inheritance patterns

Fragile X syndrome is an X-linked dominant condition. This means that fragile X syndrome can be passed from a carrier mother to her children. Fathers with the fragile X syndrome cannot transmit it to any of their sons, as fathers pass only Y chromosome to their sons. However, fathers will transmit the pre-mutation to all of their daughters, as males pass their X chromosome to all of their daughters.

Females that have one copy of the fragile X can transmit it to their sons or daughters. Each child will have a 50% of chance to inherit the fragile X. Sons who receive the fragile X will be more prone of developing intellectual disability. Daughters who receive the fragile X may even appear normal or be moderately intellectually disabled, usually to a lesser degree than male offspring.

Inheritance patterns of fragile X syndrome are very complicated and each person with a history of fragile X syndrome should consult a medical geneticist or a genetic counselor to learn more about the risks.

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