Genetic abnormality in fragile x syndrome-
Fragile X syndrome is the prototype of diseases in which the mutation is characterized by a long repeating sequence of 3 nucleotides.
These mutations are characterized by amplification of a sequence of three nucleotides.
Although the specific nucleotide sequence that undergoes amplification differs in various disorders, almost all affected sequences share the nucleotides Guanine (G) and cytosine (C).

Also, know -

It is the second most common cause of mental retardation after Down's syndrome.
It is an X linked disorder characterized by an inducible cytogenetic abnormality in the X chromosome and an unusual mutation within the familial mental retardation (-1 FMR-1 gene).

Trinucleotide Repeat disorders

Expansions affecting the non-coding region
Fragile X syndrome
Frederich's ataxia
Myotonic dystrophy

Expansions affecting coding region

Spinobulbar muscular atrophy
Huntington's disease
Dentatorubral pallidoluysian
Spinocerebellar ataxia type 1
Spinocerebellar ataxia type 2
Spinocerebellar ataxia type 3
Spinocerebellar ataxia type 6
Spinocerebellar ataxia type 7
Trinucleotide repeats
Point mutation
Not inheritable
Deletion