The 17 March 2005 issue of Nature (Vol. 434, No. 7031, pp 325-337 and pp. 400-404) reveals the genetic secrets of one of the most intriguing of human chromosomes: the X chromosome.
Female mammals carry two X chromosomes whereas males carry an X and a Y. This means that disease-causing genes on the X chromosome, including those causing haemophilia and X-linked muscular dystrophy, mainly cause problems in males who possess only a single gene copy. To ensure that females don't get a double dose of X chromosome genes, one copy is randomly silenced in each cell.
Mark Ross and his colleagues determined the sequence of over 99% of the gene-containing region of the X chromosome. The code reveals how the X and Y chromosomes evolved from a pair of regular chromosomes some 300 million years ago. Among other discoveries, the team predicts that nearly 10% of the 1,098 genes on the X chromosome are in a class that is upregulated in testicular and other cancers, and that a particular type of repetitive sequence takes up one-third of the entire chromosome and may to help silence the X chromosome's genes.
In a second paper that surveys X-chromosome-inactivated genes in more detail, Laura Carrel and Huntington Willard show that 75% of genes are silent all the time, about 15% escape inactivation and the remaining 10% are inactive on some X chromosomes but not others. This suggests that females differ widely in their pattern of gene activity, something that was previously unsuspected.
Mark Ross (Sanger Institute, Hinxton, Cambridgeshire, UK) Paper 
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Laura Carrel (Penn State College of Medicine, Hershey, PA, USA) Paper 
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Huntington F. Willard (Duke University, Durham, NC, USA) Paper 
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Chris Gunter (Senior Editor, Nature, Washington, DC, USA) News and Views author
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Message posted by: Trevor M. D'Souza