X-linked recessive inheritance
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Fragile X syndrome

Fragile X Syndrome is the most common inherited cause of mental retardation, and is associated with autism. more...

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The fragile X syndrome is a genetic disorder caused by mutation of the FMR1 gene on the X chromosome. Mutation at that site is found in 1 out of about every 2000 males and 1 out of about every 4000 females.

Normally, the FMR1 gene contains between 6 and 53 repeats of the CGG codon (trinucleotide repeats). In people with the fragile X syndrome, the FMR1 allele has over 230 repeats of this codon.

Expansion of the CGG repeating codon to such a degree results in a methylation of that portion of the DNA, effectively silencing the expression of the FMR1 protein.

This methylation of the FMR1 locus in chromosome band Xq27.3 is believed to result in constriction and fragility of the X chromosome at that point, a phenomenon that gave the syndrome its name.

The mutation and methylation of the FMR1 gene lead to the transcriptional silencing of the fragile X-mental retardation protein, FMRP. In normal individuals, FMRP binds and facilitates the translation of a number of essential neuronal RNAs. In fragile X patients, however, these RNAs are not translated into proteins. The various sequelae of fragile X syndrome result.

Transmission of the Fragile X

The diagram (above) of X-linked recessive inheritance is not entirely inappropriate but it markedly oversimplifies the situation and does not provide a sufficient foundation for genetic counseling with the fragile X syndrome.

Because males normally have only one copy of the X chromosome, those males with significant trinucleotide expansion at the FMR1 locus are symptomatic. They are mentally retarded and may show various physical features of the fragile X syndrome.

Females have two X chromosomes and thus have double the chance of having a working FMR1 allele. Females carrying one X chromosome with an expanded FMR1 gene can have some signs and symptoms of the disorder or be normal.

Males with the fragile X cannot transmit it to any of their sons (since males contribute a Y chromosome, not an X, to their male offspring.)

Females carrying one copy of the fragile X can transmit it to their sons or daughters. Sons who receive the fragile X are at high risk for mental retardation. Daughters who receive the fragile X may appear normal or they may be mentally retarded, usually to a lesser degree than boys with the syndrome.


Aside from mental retardation, prominent characteristics of the syndrome include an elongated face, large or protruding ears, large testicles (macroorchidism), and low muscle tone. Behavioral characteristics may include stereotypic movements (e.g., hand-flapping) and atypical social development, particularly shyness and limited eye contact. Some individuals with the fragile X syndrome also meet the diagnostic criteria for autism.

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Fragile X Syndrome
From Gale Encyclopedia of Childhood and Adolescence, 4/6/01

Fragile X syndrome is a genetic disorder that occurs in all ethnic groups, and one out of 700 pregnant women carries the fragile X defect. In 1992, a new test was announced for determining whether a woman carries the fragile X genetic defect. When a woman learns that she carries this defect, she may then elect to undergo amniocentesis or chorionic villus tests to determine whether her fetus also carries the fragile X defect. The incidence of fragile X syndrome is estimated to be approximately one in every 2,000 to 3,000 births; the estimates include one per 1,500 males and one per 2,500 females.

Fragile X appears to be caused by an abnormal number of repeats of a genetic sequence on a segment of the X chromosome. (It was initially characterized as the Martin-Bell syndrome.) Because fragile X has been observed in all ethnic groups, it may be considered one of the most frequent single-gene disorders in humans. It is difficult to diagnose in infants, but the features slowly become evident as the child grows.

Developmental delay and mental retardation are the most significant features of fragile X syndrome. In fact, fragile X syndrome is a common cause of mental retardation--researchers estimate that 20% of all boys with IQ levels between 30 and 55 (severely to moderately retarded) have fragile X syndrome. Overall, the population with fragile X syndrome ranges in mental retardation from profound (IQ below 25) to borderline, with an average IQ (intelligence quotient) in the moderately retarded range (35-55).

Before the onset of puberty, boys with the syndrome have delayed developmental milestones and may display some avoidance behavior similar to autism. They may also exhibit hyperactivity and attention deficit/hyperactivity disorder . Language delay is frequently observed; absence of speech is rare, but a playful, repetitive speech pattern is common.

An adult male with fragile X syndrome is likely to have the following characteristics: a long, narrow face with head circumference above the 50th percentile; prominent jaw and forehead; large and slightly malformed ears; hyperextending joints; high arched palate, and enlarged testicular volume.

In girls, the symptoms appear to be milder than in boys. The mental retardation is less severe, with most female patients falling in the mild-to-borderline retarded range. Emotional problems are reportedly more common in females with fragile X syndrome. An adult female with fragile X syndrome has facial characteristics similar to an adult male--prominent jaw and forehead and large, protruding ears.

Mental retardation caused by fragile X syndrome (or any other cause) often requires that the individual receive lifetime custodial care.

Gale Encyclopedia of Childhood & Adolescence. Gale Research, 1998.

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