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Fragile-X syndrome: Unique genetics of the heritable unstable element
Authors:S Yu  J Mulley  D Loesch  G Turner  A Donnelly  A Gedeon  D Hillen  E Kremer  M Lynch  M Pritchard  G R Sutherland  and R I Richards
Affiliation:Department of Cytogenetics and Molecular Genetics, Adelaide Children's Hospital, Australia.
Abstract:The fragile site at Xq27.3 is an unstable microsatellite repeat, p(CCG)n. In fragile-X syndrome pedigrees, this sequence exhibits variable amplification, the length of which correlates with fragile-site expression. There is a direct relationship between increased p(CCG)n copy number and propensity for instability: individuals having large amplifications exhibit somatic variation due to increased instability. The instability of the p(CCG)n repeat, when transmitted through affected pedigrees, explains the unusual segregation patterns of fragile-X phenotype, referred to as the Sherman paradox. All individuals of fragile-X genotype were found (where testing was possible) to have a parent with amplified p(CCG)n repeat, indicating that few, if any, cases of fragile-X syndrome are not familial.
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