<prompt> Genetic abnormality in fragile X-syndrome: PGI June 2007Trinucleotide repeate disorder.Point mutationNot inheritableDeletion  <response>: Ans.: A Trinucleotide repeat disorder Fragile X syndrome is the prototype of the disease in which the mutation is characterized by a long repeating sequences of three nucleotidesRobbins 7th 181Fragile X syndromeSecond most common genetic causes of mental retardation , after Down syndromeThe only distinctive feature that can be detected in at least 90 of postpubertal males with fragile -X syndrome is macro-orchidismc-Long face with a large mandible large everted earsHyperextensible joints, a high arched palate mitral valve prolapseQ is noted in some patientsAs with all X-linked disease. Fragile X syndrome affects malesTable: Selected Trinucleotide Repeat DisordersDiseaseLocusRepeatTriplet Length NormalDiseaseInheritanceGene ProductX-chromosotnal spinobulbar muscular atrophy SBMAXq11-q12CAG11-3440-62XRAndrogen receptorFragile X-syndrome FRAXAXq27.3CGG6-50200-300XRFMR-1 proteinFragile X-syndrome FRAXEXq28GCC6-25200XRFMR-2 proteinDystrophia myotonica DM19ql3.2-q13.3CTG5-30200-1000AD, variable penetranceMyotonin protein kinaseHuntington disease H D4pl6.3CAG6-3437-180ADHuntingtonSpinocerebellar ataxia type 1 SCA16p213-21.2CAG6-3940-88ADAtaxin 1Spinocerebellar ataxia type 2 SCA212q24.1 ACAG15-3134-400ADAtaxia 2Spinocerebellar ataxia type 3 SCA3; Machado Joseph disease MD14q21CAG13-3655-86ADAtaxin 3Spinocerebellar ataxia type 6 SCA6, CACNAIAI9pl 3.1-13.2CAG4-1620-33ADAlpha 1A voltage- dependent L-type calcium channelSpinocerebellar ataxia type 7 SCA73p21.1-pl2CAG4-1937 to 300ADAtaxin 7Spinocerebellar ataxia type 12 SCA125q31CAG2666-78ADProtein phosphatase 2 ADentorubral pallidoluysiane atrophy DRPLA12pCAG7-2349-75ADAtnopbin 1Friedreich ataxia FRDA19ql3-21GAA7-22200-900ARFrataxinNote: AD, autosomal dominant; AR, autosomal recessive; XR. X-linked recessive. 