**2.2 Evolution of genomic imprinting**

Most autosomal genes in diploid organisms are expressed from both the maternal and paternal copies at equal levels. However, there are roughly 80 exceptional genes in eutherian mammals that are monoallelically expressed in a parent of origin fashion. The silent allele is marked (imprinted) by epigenetic features, such as CpG methylation and histone modifications. The evolution of a genomic imprinting mechanism appears counterintuitive since surely it would be more advantageous to have two expressed copies of a gene to protect the individual against deleterious mutations occurring in one copy. Consequently, genomic imprinting raises many questions regarding the how and why genomic imprinting evolved, although there appears to be some link between the evolution of viviparity and genomic imprinting (Hore et al., 2007).

By examining the orthologues of eutherian imprinted genes in marsupials and monotremes, it becomes possible to begin addressing some the questions regarding the evolution of genomic imprinting. Gene mapping with BAC probes and BAC clone sequencing have contributed greatly to research in this area. Below are just a few examples where the use of BAC clones has proven critical for tracing the evolutionary history of imprinted loci. Even in the fairly well covered opossum genome sequence, it has been necessary to sequence BAC clones spanning regions of interest in order to fill gaps in the genome assembly. Major conclusions drawn from these studies propose that imprinting arose independently at each imprinted locus and that the acquisition of imprinting involved changes to the genomic landscape of the imprinted region.
