Fragile X-syndrome is characterized by all of the following except -

Fragile X - syndrome It is caused by Triplet repeat mutations, the mutations are characterized by a long repeating sequence of three nucleotides. It is an X-linked disorder characterized by an inducible cytogenetic abnormality in the X-chromosome. The cytogenetic abnormality is seen as a discontinuity of staining or as a constriction in the long arm of the X chromosome. Because it appears that the chromosome is "broken" at this locale, it is referred to the as fragile site. Clinical features Fragile X syndrome is the second most common genetic cause of mental retardation, after Down's syndrome. In this syndrome, the affected males are mentally retarded with I.Q. in the range of 20 - 60. These patients express a characteristic phenotype that includes Long face with long mandible Large everted ears Large testicles (macroorchidism) Hyperextensible joints, high arched palate and mitral valve prolapse. These and other physical abnormalities described in this condition, however, are not always present and in some cases are quite subtle. The only distinctive feature that can be detected in at least 90% of postpubertal males with fragile X-syndrome is macro-orchidism. As with all X linked diseases, fragile X syndromes affects males.