Figure 13

The f₂ values for putative ortholog pairs in rat and mouse are higher if they lie on the X chromosome (panel (b), mean f₂ ≈ 0.93) than pairs on autosomal chromosomes (panel (a), mean f₂ ≈ 0.90), implying that the X chromosome genes have accumulated fewer silent transitions at two fold redundant sites than the typical pair of orthologs. Since fewer than 5% of the genes lie on the X chromosome, this can account for only some of the overdispersion in the f₂ values for rat-mouse orthologs. Interestingly, an analogous phenomenon was not observed in human-canine ortholog pairs (data not shown).