Y not X?

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Science  18 Apr 2008:
Vol. 320, Issue 5874, pp. 291
DOI: 10.1126/science.320.5874.291b

In mammals, a disproportionately high number of genes from the X chromosome have been moved, via transposable elements, to the autosomes. By examining all retrotransposed genes in three placental and one marsupial mammal, Potrzebowski et al. have found that only a subset of these genes, the majority originating from the X chromosome, have retained their functions. In mice, most of the relocated genes originating from the X chromosome are specifically expressed in the testes, while those originating from other chromosomes are expressed broadly. The movement is due to a process known as meiotic sex chromosome inactivation, where replicated genes take over the function of their X-linked parents, which are silenced during meiosis. On the basis of these results, the authors hypothesize that the origin of these meiotic retrogenes dates to the divergence of the placental and marsupial mammals. Because meiotic X inactivation reflects the differentiation of the X and Y chromosomes, these chromosomes may be younger than previously suggested. — LMZ

PLoS Biol. 6, e80 (2008).

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