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What is Which is not true about Fragile X syndrome? and explain in detail?
Fragile - X syndrome 

Fragile - X syndrome is the prototype of diseases in which the mutation is characterized by a long repeating sequence of three nucleotides.
In fragile X syndrome, trinucleotide repeat mutation involves CGG on the non-coding region.

Clinical features of fragile - X syndrome

Mental retardation  
Long face with large mandible
Hyperextensible joint 
Mitral valve prolapse
Large everted ears
Large testis (macroorchidism) 
High arched palate       
Fragile X syndrome is the second most common cause of mental retardation, after Downs syndrome.