The majority of deleterious mutations are thought to be recessive Crow and Temin ; Mukai et al. Baker, Tobias, and Varshavsky Open in new tab Download slide. However, there is conflicting evidence as to whether the evolutionary rate of the X chromosome is accelerated or reduced compared with that of autosomes. Two of these genes, cylicin I and actin-related protein T1, may be important in sperm head structure, which may affect a sperm's ability to fertilize the egg. First, reduced polymorphism on the X chromosome has been reported in Drosophila simulans , suggesting that a form of positive selection may be acting on sex chromosomes Begun and Whitley Two studies in mammals support this theory and have found X-linked genes to have a significantly lower nonsynonymous substitution rate Wolfe and Sharp ; McVean and Hurst Select Format Select format. Sexual selection can influence sex gene-pool evolution in terms of broad-sense sexual selection and may not be limited to secondary sexual traits and mating behavior Civetta and Singh
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