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A new scientific study is challenging familiar ideas about genetic inheritance. We can't change the genes we got from our parents, but our DNA is controlled by a kind of chemical instruction manual. And those instructions can be rewritten by our circumstances; for example, by obesity. According to the study, these rewrites can even be passed along to our children.

NPR's Dan Charles has the story.

DAN CHARLES: Margaret Morris studies overweight people. She's a researcher at the University of New South Wales in Australia. Recently, she was looking into reasons why the children of obese mothers often become overweight themselves. But then, a new graduate student arrived in her lab from Malaysia.

Professor MARGARET MORRIS (Obesity Researcher, University of New South Wales): And she noted in the clinic that when a child arrived for weight management, usually both parents were obese, not just the mother.

CHARLES: This wasn't too surprising. It makes sense. If a father is genetically predisposed to obesity, his daughter might be too. But Morris wondered whether she might be seeing more than genes at work. She set up an experiment with lab rats to see if the biological consequences of a father overeating could somehow get passed on to his daughters.

Prof. MORRIS: This is a study I did for love. I didn't really have much funding for it. I had to crib money from all over to do it.

CHARLES: She took a group of genetically identical male rats and put half of them on a high-fat diet. Predictably, these rats got fat and suffered symptoms of diabetes. Then, all the rats mated with normal females and had children. Morris looked specifically at the daughters. All of them had the same genetic makeup, but those with overweight fathers had some of the same problems their dads did. They weren't overweight, but their production of insulin was impaired.

Dr. ANDY FEINBERG (Director, Center for Epigenetics, Johns Hopkins School of Medicine): In a way, it's saying, like, the metabolic sins of the fathers can be visited on the daughters even if the daughter hasn't been conceived yet.

CHARLES: Andy Feinberg is at the Johns Hopkins School of Medicine in Baltimore. He's director of its center for epigenetics. That word - epigenetics - is getting a lot more attention these days. It refers to things in and around our DNA - billions of chemical marks that attach to it, for instance. Those marks are signals that turn genes on and off. They tell the genes of a liver cell to behave differently from genes in a blood cell, for instance. The sequence of our DNA - the human genome - has been called the book of life. Feinberg has his own metaphor for the billions of added signals that he studies.

Dr. FEINBERG: I would consider the genome to be the words and the epigenome to be the grammar.

CHARLES: The grammar?

Dr. FEINBERG: Yeah.

(Soundbite of laughter)

CHARLES: How the grammar?

Dr. FEINBERG: Well, because it helps to tell what the genes are actually supposed to do and puts them into context.

CHARLES: Our genes don't change. Or if they do, it's rare and random. But the grammar of the epigenome is changing all the time. It can also be disrupted by chemicals we eat or breathe and, evidently, by obesity, because Feinberg thinks those fat dad rats in Australia created sperm cells with a different pattern of epigenetic marks on their DNA. That's how the effect showed up in their children. The study appears in this week's issue of the journal Nature.

Michael Skinner at Washington State University says epigenetic effects are swinging the pendulum of scientific attention from the genetic code back toward the impact of environment.

Dr. MICHAEL SKINNER (Professor, School of Molecular Biosciences, Washington State University): That's probably through more epigenetic mechanisms. There's also a baseline genetic element of biology. And the two combined will actually give us more information about how things work.

CHARLES: Much of epigenetics is still a mystery. Scientists would like to know, for instance, how often epigenetic signals are passed on from parent to child or even to grandchild. So Margaret Morris, in Australia, is hoping to repeat her experiment and see if the effect persists over multiple generations.

Dan Charles, NPR News, Washington.

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