Fragile X syndrome

Fragile X syndrome an X-linked disorder caused by expansion of the CGG trinucleotide repeat affecting the Fragile X mental retardation 1 (FMR1) gene on the X chromosome. X-linked dominant – males are more severely affected by this disorder than females. It is the most common inherited form of mental retardation and second only to Down’s syndrome as a cause of mental impairment.  Mothers of affected patients are obligate gene carriers.


  • Learning  disabilities (mental retardation) and cognitive impairmentWikimedia Image author: Peter Saxon
  • Language delay, echolalia
    • Large, protruding ears
    • Long face
    • Large Jaw
    • Large testes (Macroorchidism) at puberty
    • Hyperextensible finger joints
    • Hyperextensible (‘Double-jointed’) thumbs


DNA probe to pinpoint the defective gene

Other Names

  • fra(X) syndrome
  • FRAXA syndrome
  • FXS
  • marker X syndrome
  • Martin-Bell syndrome
  • X-linked mental retardation and macroorchidism

MCQ  Point

Fragile X syndrome is associated with the following characteristics)  

a) More common in boys
b)  Most common form of inherited mental retardation
c)  Involves the long arm of the X chromosome
d)  Can be diagnosed with a blood test
e)  Treatment is limited to symptomatic management