**2.1 Origins of marsupial and monotreme sex chromosomes**

Determining the evolutionary origins of marsupial and monotreme sex chromosomes was the driving force behind much of the gene mapping conducted in these species. The earliest gene mapping work showed that at least some genes found on the human X chromosome were also on the X in marsupials, resulting in the hypothesis that the X chromosome of these two mammalian groups had a common origin. Gene mapping using heterologous probes and radioactive in situ hybridization (RISH) supported the extension of this hypothesis to include monotremes. However, it was only when BAC clones became available for gene mapping that the true evolutionary history of sex chromosomes in these species was revealed.
