Australia: The Land Where Time Began

A biography of the Australian continent 

Anatomy Based relationship - X-chromosome inactivation

In mammals the females have 2 X chromosomes, 1 from each parent, each of which has the same complement of genes, so the females get 2 of every gene, whereas the males have a single copy of each X chromosome gene, that invariably came from their mother. One of the X chromosomes is inactivated in the females of both marsupials and placentals, and replicates later than the active X chromosome. The X chromosome that is inactivated, in any cell, may be from either parent in placentals. Hence the term random X inactivation. In marsupials it is nearly always the X chromosome from the father that is inactivated - paternal X inactivation. This aspect of marsupial genetics has been studied most in the kangaroos, but some evidence exists for this process occurring in other families. Most tissues in bandicoot bodies have only 1 X chromosome, during development, in females the second X and the Y chromosome in males. The full sex chromosome complement is retained only in the tissues of the gonads (Hayman, 1990) - sex chromosome mosaicism.

The chromosome number and arrangement supports the relationships among marsupials that have been determined based on other criteria, but the many changes in the number and order of the chromosomes mean it hard to interpret, being of more use to support conclusions that are made based on other criteria, such as protein and DNA sequences.

1. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
2. M.Archer, S.J. Hand & H. Godthelp in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
3. Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.