True about Fragile X syndrome: (PGI June 2009)

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 7th/181"Fragile sitesQ are region of the chromosome that show a tendency for separation, breakage or attenuation under particular growth conditions . One fragile site(of 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 18th/515Fragile 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 Length(Normal/Disease)InheritanceGene ProductX-chromosomal spinobulbar muscular atrophy (SBMA)Xq11-q12CAG11-34/40-62XRAndrogen receptorFragile X-syndrome (FRAXA)Xq27.3CGG6-50/200-300XRFMR-1 proteinQFragile X-syndrome (FRAXE)Xq28GCC6-25/>200XRFMR-2 protein QDystrophia mvotonica (DM) 519ql3.2-ql3.3CTG5-30/200-1000AD, variable penetranceMyotonin protein kinaseHuntington disease (HD)Q4pl6.3CAG6-34/37-180ADHuntington QSpinocerebellar ataxia type 1 (SCA1)6p21.3-21.2CAG6-39/40-88ADAtaxin 1QSpinocerebellar ataxia type 2 (SCA2)12q24.1CAG15-31/34-400ADAtaxin 2Spinocerebellar ataxia type 3 (SCA3); Machado Joseph disease (MD)14q21CAG13-36/55-86ADAtaxin 3Spinocerebellar ataxia type 6 (SCA6. CACNAIA)19pl3.1-13.2CAG4-16/20-33ADAlpha 1A voltage- dependent L-type calcium channelSpinocerebellar ataxia type 7 (SCA7)3p21.1-pl2CAG4-19/37 to >300ADAtaxin 7Spinocerebellar ataxia type 12 (SCA12)Sq31CAG6-26/66-78ADProtein phosphatase 2ADentorubral pallidoluysiane atrophy (DRPLA)12pCAG7-23/49-75ADAtrophin 1Friedreich ataxia (FRDA1)9ql3-21GAA7-22/200-900ARFrataxinNote: AD. autosomal dominant: AR, autosomal recessive; XR, X-linked recessive.