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Since Haldane first noticed an excess of paternally derived mutations, it has been considered that most mutations derive from errors during germ line replication. Miyata et al. (1987) proposed that differences in the rate of neutral evolution on X, Y, and autosome can be employed to measure the extent of this male bias. This commonly applied method assumes replication to be the sole source of between-chromosome variation in substitution rates. We propose a simple test of this assumption: If true, estimates of the male bias should be independent of which two chromosomal classes are compared. Prior evidence from rodents suggested that this might not be true, but conclusions were limited by a lack of rat Y-linked sequence. We therefore sequenced two rat Y-linked bacterial artificial chromosomes and determined evolutionary rate by comparison with mouse. For estimation of rates we consider both introns and synonymous rates. Surprisingly, for both data sets the prediction of congruent estimates of is strongly rejected. Indeed, some comparisons suggest a female bias with autosomes evolving faster than Y-linked sequence. We conclude that the method of Miyata et al. (1987) has the potential to provide incorrect estimates. Correcting the method requires understanding of the other causes of substitution that might differ between chromosomal classes. One possible cause is recombination-associated substitution bias for which we find some evidence. We note that if, as some suggest, this association is dominantly owing to male recombination, the high estimates of seen in birds is to be expected as Z chromosomes recombine in males.