Figure 1.

Previously proposed scheme for the origin of the human X and Y chromosomes. The three evolutionary strata (Kohn et al. 2004) and the recently added (XAR, red) and conserved (XCR, dark blue) regions of the human X and Y chromosomes are shown. The XAR and XCR regions are shown on the chromosomes (Chr) of the tammar wallaby, platypus, and chicken, but the location of stratum 2 (light blue) in the tammar wallaby and platypus is unknown. The existence of homologs of stratum 2 genes on chicken chromosome 12 is evidence for a separate evolutionary origin for this stratum (Kohn et al. 2004). Divergence times given between each of the groups are millions of years ago.