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In the last few years, epigenetics has seized the public imagination. Magazines like New Scientist are publishing articles with titles such as "How to change your genes by changing your lifestyle". 1 A book called "The Epigenetic Revolution" has become a bestseller. 2 The popular press regularly reports epigenetic research, including a recent study purporting to show that severe psychological trauma can be passed down through one's genes. 3 Overall, it would be easy for anyone nowadays to form the impression that the genetic destiny of future generations is entirely in our own hands, DNA is highly malleable, and Darwin and Mendel have been thoroughly discredited. In this article, I want to try and bring epigenetics back down to earth, with a simple account of what it is and is not, and what epigenetic research has and has not shown so far.
Epigenetics is best defined as the study of changes in organisms brought about by modification of gene expression, rather than by alteration of the genetic code in the form of DNA. The term "epigenetic" was originally coined in the 1950s, and used to denote the way that genes interact with the environment, in order to produce each individual phenotype. To describe this process at its simplest: you may be born with a capacity to be tall and confident, but if you are undernourished and abused as a child, you are likely to turn into a stunted and fearful adult instead. There is nothing very remarkable about the way genes interact with the environment in this way. Concepts like gene expression and plasticity have been commonplace in biology for a long time, and indeed form the basis of all modern biological thinking.
Since genes alone do not determine phenotypes, mechanisms have to exist at the molecular level in order to mediate gene-environment interactions. As soon as some of these mechanisms were discovered, the term "epigenetics" came to be applied to them as well. The best known of these is methylation, where a methyl group binds to cytosine on a stretch of DNA, and renders it less active. The mechanisms themselves are very common in nature, quite aside from environmental influences, and they govern gene expression in all kinds of ways, including turning a stem...