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Biol Lett. Jun 23, 2009; 5(3): 409–412.
Published online Jan 14, 2009. doi:  10.1098/rsbl.2008.0732
PMCID: PMC2679909
Are sex-biased genes more dispensable?
Judith E. Mank1,2* and Hans Ellegren2
1Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
2Department of Evolutionary Biology, Evolutionary Biology Centre, Uppsala University, Uppsala 752 36, Sweden
*Author for correspondence (judith.mank/at/
Received December 4, 2008; Accepted December 9, 2008.
Many genes show different expression levels in males and females, and these form the basis of sexually dimorphic phenotypes. Sex-biased genes experience accelerated rates of protein evolution, which has been attributed to sexual selection. However, it is possible that the increased rates of molecular evolution, and more importantly the sex-biased gene expression pattern itself, are due to decreased selective constraint. This notion may explain many of the patterns associated with sex-biased gene expression, and changes how we should view the role of natural and sexual selection in relation to these genes.
Keywords: gene expression, positive selection, relaxed constraint
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