Figure 4. Fixed differences in X-linked genes are not uniformly distributed across the X.

There are 955 fixed differences on the X chromosome between the X^SR and X^ST transcriptomes across 27% of the X-linked transcripts. We compared the observed counts of transcripts with various numbers of fixed differences (blue line) to the expected distribution of fixed differences across genes if fixed differences were distributed randomly across these genes using draws from the binomial distribution with fixed differences appearing at a rate proportional to the observed per-basepair rate of fixed differences on the X (black) or autosomes (red). Compared to the X expectation there was an excess of X-linked transcripts with zero fixed differences (A). There was also an excess of transcripts with six or more fixed differences (B). Together these data suggest that fixed differences on the X are clustered non-uniformly with some transcripts having more fixed differences than expected.