Epigenetics is the study of the heredity of variations in gene regulation and, thus, phenotype, that occurs independent of any change in DNA sequence. The primary mechanism by which this happens is through post translational modification (e.g. methylation and acetylation) of histones, although several other mechanisms are known to exist. Epigenetic gene regulation is also important physiologically; epigenetic reprogramming is one of the major mechanisms by which pluripotent stem cells and, subsequently, their daughter cells are committed to a specific cell type lineage during development.
Epigenetic inheritance allows for more dramatic and rapid phenotypic change withinin populations across generations, and thus more rapid adaptation to environmental change, than random mutation alone. In some species, certain traits are known to be on a sort of epigenetic "on/off switch" (e.g. the presence or absence of protective spines in Raphanus raphanistrum, the wild radish), where they can be activated or deactivated between generations in response to environmental challenge.
Epigenetic silencing is also thought to provide a natural protection against retroviruses and inappropriate activation of transposable DNA elements (segments of DNA that move around in the genome). Although, the precise nature of this role is still somewhat disputed and remains an active area of research.
However, the most important role of epigenetic gene regulation, by far, is in differential gene regulation during development, where epigenetic reprogramming is a major regulator of cell differentiation.
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- Simmen et al. (1999). Nonmethylated transposable elements and methylated genes in a chordate genome. Science 283, 1164-1167.