Tufts OpenCourseware
Authors: John Morgan, Bonnie F. Zimble

Fragile X Syndrome:

What is Fragile X Syndrome? Characteristics

Special Care in Dentistry
John Morgan, DDS
Tufts University School of Dental Medicine, 2008


Study Questions 

1.     Describe Fragile X Syndrome.

2.     What are the three forms of prenatal testing for Fragile X Syndrome?

3.     Comment on the following characteristics of Fragile X Syndrome:

a.      Physical characteristics

b.     Behavioral characteristics

c.      Mental impairment

d.     Medical problems 

What is Fragile X Syndrome?

Fragile X syndrome is the most common genetically inherited form of mental retardation currently known. In addition to intellectual disability, some individuals with Fragile X display common physical traits and characteristic facial features, such as prominent ears.  This genetic syndrome is caused by a defect on the X chromosome. Fragile X exhibits X-linkage.  The effect of X-linkage is that the frequency of the syndrome is greater in males than females.  The observable characteristics of Fragile X occur in approximately 1:1,000 male births and 1:2,500 female births.   

Click here to access the National Institute of Child Health and Human Development (NICHD)  web site and learn more about Fragile X Syndrome.

 Summary information follows ... 

Inheritance. The Fragile X mutation appears to increase in length as it is inherited by succeeding generations.  This phenomenon is known as “genetic anticipation.”

Testing. A simple test is now available that can determine if a woman is carrier of the Fragile X gene.  A drop of blood can be taken from the woman’s finger and analyzed quickly and inexpensively.

Prenatal testing.
Three prenatal tests can determine if Fragile X is present in the fetus:

    • Chorionic villi sampling (CVS): involves extracting a tiny amount of fetal tissue at 9 to 11 weeks of pregnancy.      

    • Amniocentesis is the removal and analysis of a small sample of fetal cells from the amniotic fluid. Amniocentesis is widely available and involves a lower risk of miscarriage.   

    • Percutaneous umbilical blood sampling (PUBS) is the most accurate method of testing, and can be used to confirm the results of CVS or amniocentesis.

Characteristics of Fragile X Syndrome

Individuals with Fragile X may have a cluster of physical, behavioral, mental, and other characteristics.

Physical Characteristics
Males with Fragile X have some common physical characteristics:  a long narrow face; large or prominent ears; and macroorchidism (enlarged testicles).  More than 80 percent of males with Fragile X develop at least one of these features, but often not until after puberty.  Other physical characteristics of males with Fragile X are double-jointed fingers, flat feet, puffy eyelids, and “hollow chest.”  These physical features may indicate an underlying abnormality of the connective tissue, although no specific connective tissue defect has been detected.  Females with Fragile X syndrome do not exhibit most of the physical characteristics found in males, although they often have large or prominent ears.

Behavioral Characteristics
The most prevalent behavioral characteristics of children with Fragile X are attention problems and hyperactivity, known as attention-deficit hyperactivity disorder (ADHD). Children with Fragile X appear to have an underlying disability related to processing external stimuli.  Some of the other behaviors associated with Fragile X are similar to those of autism. One strength of males with Fragile X is their great sociability and friendliness. 

Mental Impairment 
Mental retardation associated with Fragile X is similar to that of Down syndrome in that most of those affected fall somewhere in the middle range of impairment.   

Medical Problems 
Most children with Fragile X do not have serious physical problems, but are at greater risk for certain types of moderate medical problems than are normal children.  

They often suffer recurrent otitis media (inner ear infections).

  • Common eye problems include myopia (nearsightedness) and a high incidence of “lazy eye.”

  • Orthopedic difficulties related to flat feet and joint laxity may occur. 

  • Twenty-percent of males with Fragile X are prone to seizures. 

  • In addition, many children with Fragile X have digestive disorders, such as gastroesophageal reflux that causes gagging, regurgitation, and discomfort.