Is increasingly recognised, especially after assisted reproduction

Genomic imprinting, defined as gene expression dependent on the parent of origin,1 has been increasingly recognised over the past decade as a mechanism contributing to human disease. The topic now features as a core part of any genetics curriculum, appears in postgraduate medical examinations, and is a term familiar to many clinicians. Recently abnormalities of genomic imprinting have been discussed in the context of assisted reproductive technologies. So what exactly is genomic imprinting, and how does it occur?

For many years a gene was assumed to have the same function, whether it was inherited from the mother or the father. We now know this is not the case, as the DNA of some genes is modified during gametogenesis and as a result may have altered expression, becoming either inactivated or activated. Genes that are susceptible to parent specific modification in this way (termed epigenetic, because the modification does not entail mutation of the DNA code) are referred to as imprinted genes.2 The mechanism of imprinting is still not entirely clear but in most cases entails the process of methylation, with imprinted genes showing differences in methylation between the …