<prompt> True about Fragile X syndrome: PGI June 2009Triplet nucleotide repeat sequenceChromosomal breakingMitochondrial mutationCentrochrome absent  <response>: Ans: A Triplet nucleotide repeat sequence Fragile X syndrome is the prototype of the disease in which the nutation is characterized by a long repeating sequences of three nucleotides- Robbins 7th181Fragile sitesQ are region of the chromosome that show a tendency for separation, breakage or attenuation under particular growth conditions . One fragile siteof total of 120 in human genome that has clinical significance is that on the distal long arm of chromosome Xql 7.3 associated with FragileXsyndrome- Nelson 18th515Fragile X Syndrome Second most common genetic causes of mental retardation , after Down syndrome The only distinctive feature that can be detected in at least 90 of postpubertal males with fragile -X syndrome is macroorchidismQ Long face with a large mandible targe everted ears Hyperextensible joints, a high arched palate mitral valve prolapse is noted in some patients As with all X-linked disease fragile X syndrome affects malesTable: Selected Trinucleotide Repeat DisordersDiseaseLocusRepeatTriplet LengthNormalDiseaseInheritanceGene ProductX-chromosomal spinobulbar muscular atrophy SBMAXq11-q12CAG11-3440-62XRAndrogen receptorFragile X-syndrome FRAXAXq27.3CGG6-50200-300XRFMR-1 proteinQFragile X-syndrome FRAXEXq28GCC6-25200XRFMR-2 protein QDystrophia mvotonica DM 519ql3.2-ql3.3CTG5-30200-1000AD, variable penetranceMyotonin protein kinaseHuntington disease HDQ4pl6.3CAG6-3437-180ADHuntington QSpinocerebellar ataxia type 1 SCA16p21.3-21.2CAG6-3940-88ADAtaxin 1QSpinocerebellar ataxia type 2 SCA212q24.1CAG15-3134-400ADAtaxin 2Spinocerebellar ataxia type 3 SCA3; Machado Joseph disease MD14q21CAG13-3655-86ADAtaxin 3Spinocerebellar ataxia type 6 SCA6. CACNAIA19pl3.1-13.2CAG4-1620-33ADAlpha 1A voltage- dependent L-type calcium channelSpinocerebellar ataxia type 7 SCA73p21.1-pl2CAG4-1937 to 300ADAtaxin 7Spinocerebellar ataxia type 12 SCA12Sq31CAG6-2666-78ADProtein phosphatase 2ADentorubral pallidoluysiane atrophy DRPLA12pCAG7-2349-75ADAtrophin 1Friedreich ataxia FRDA19ql3-21GAA7-22200-900ARFrataxinNote: AD. autosomal dominant: AR, autosomal recessive; XR, X-linked recessive. 