Epigenetics describes DNA modifications that change gene expression without altering the genetic code. Basically the structure of the DNA changes so that the genes for digestive enzymes aren’t active in the brain, for example. This is how cells with the same DNA can have different structures and functions – my skin cell differs from my pancreatic cell not because it has a different genetic code but because modifications to the DNA mean that some genes are switched off.
One of my favourite examples is tortoiseshell cats. In each of their cells one X chromosomes is inactivated by epigenetic changes. One of their chromosomes contains the genes for ginger fur, one for dark fur, and so cells produce different colour fur depending on which X chromosome is inactive. Tortoiseshells are all female because males don’t have two X chromosomes.
Epigenetics is one of the reasons that it is hard to distinguish to what extent you are shaped by your genes or your environment. You can inherit epigenetic changes, so some changes to your DNA caused by your parents’ or grandparents’ environment can be passed on to you.
Here is an introduction from the wonderful Nessa Carey: