The Mysterious X

Carl Zimmer looks into what's known about the X chromosome.

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I think the name came from

I think the name came from the X factor...

Actually the single X *is*

Actually the single X *is* upregulated in males, to compensate for the loss of homologs of X-linked genes from the Y chromosome. However, in mammals, this upregulation of the X didn't happen in a male-specific fashion, so it became necessary to inactivate one of the two X chromosomes in females to bring gene expression back into line with ancestral levels. Christine Disteche's lab has done the clearest and best work defending this hypothesis, which dates back to Susumu Ohno's monograph on sex chromosomes in 1967.