Figure 5.

Summary of the current understandings of how and when the human sex chromosomes evolved. The XCR and XAR of the human sex chromosomes are shown and their corresponding regions of the tammar wallaby, platypus, and chicken. The proposed stratum 2 is now known to be a part of the XCR of the human X chromosome and does not have a separate evolutionary origin. Our localization of four Xp11 orthologs to chicken 4p suggests that stratum 2 genes are located on chicken chromosome 4p, although the majority of these genes were unable to be mapped. Divergence times given between each of the groups are millions of years ago.