The research from epigenetics — the study of how social (and other) environments turn our genes on and off — surprises many people who believe that the DNA we’re born with doesn’t change and programs all that we are during our entire lifetime.
Here’s the reality: Toxic stress can alter genes and cause long-term changes in all parts of our bodies and brains. And these changes can be transferred from generation to generation. Essentially, a process called methylation – the addition and subtraction of tiny groups of chemicals – can turn genes on and off.
http://en.wikipedia.org/wiki/Epigenetics