Originally posted on Why Evolution Is True:
The word “epigenetics” once meant simply “development”—that is, the way the genome worked itself into an organism through the production and regulation of proteins and absorption of food and materials from the outside, and the turning of some genes on and others off in different tissues. Now, however, the term means roughly “forms of inheritance that rest on modification of the DNA sequence,” and by “DNA sequence” I mean the sequence of four bases (A, G, C, and T) that constitutes the DNA code.
We now realize, though, that some DNA bases can be modified, and in an inherited way, in a manner that can affect the development, behavior, or structure of an organism. Such modification often takes place via DNA methylation, in which some of those four bases acquire methyl groups, thereby changing how the DNA functions.
Such methylation, as you’ll see by reading the Wikipedia link above, is important in organismal development—something we’ve realized only in recent decades. For example, there is differential “imprinting” of DNA via differential methylation in male versus female parents, and this results in the DNA in the zygote doing different things depending on whether it came from mother or father (organisms have paired chromosomes, getting one from each parent). This has led to speculations about the evolution of differential imprinting resulting from different interests of mother and father in how and which zygotes develop.