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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
  • R. I. Richards
Publication Date
May 01, 1992
Source
PMC
Keywords
Disciplines
  • Biology
License
Unknown

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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