inherited from the mother is silenced, while the copy inherited from the father is active (or vice versa). The silent copy is inactive through processes involving DNA methylation. These changes all involve epigenetic processes parallel to those currently attracting so much attention. However, the difference is that these known epigenetic modifications (cell specialization, X inactivation, genomic imprinting) all occur early in development and are stable. The discovery that epigenetic modifications continue to occur across development, and can be reversible and more dynamic, has represented a major paradigm shift in our understanding of environmental regulation of gene expression.
