are we recipients of our DNA, or caretakers of it?

WaPo | Two mice. One weighs 20 grams and has brown fur. The other is a hefty 60 grams with yellow fur and is prone to diabetes and cancer. They're identical twins, with identical DNA.

So what accounts for the differences?

It turns out that their varying traits are controlled by a mediator between nature and nurture known as epigenetics. A group of molecules that sit atop our DNA, the epigenome (which means "above the genome") tells genes when to turn on and off. Duke University's Randy Jirtle made one of the mice brown and one yellow by altering their epigenetics in utero through diet. The mother of the brown, thin mouse was given a dietary supplement of folic acid, vitamin B12 and other nutrients while pregnant, and the mother of the obese mouse was not. (Though the mice had different mothers, they're genetically identical as a result of inbreeding.) The supplement "turned off" the agouti gene, which gives mice yellow coats and insatiable appetites.

"If you look at these animals and realize they're genetically identical but at 100 days old some of them are yellow, obese and have diabetes and you don't appreciate the importance of epigenetics in disease, there's frankly no hope for you," Jirtle says.

He offers this analogy: The genome is a computer's hardware, and the epigenome is the software that tells it what to do.

Epigenomes vary greatly among species, Jirtle explains, so we cannot assume that obesity in humans is preventable with prenatal vitamins. But his experiment is part of a growing body of research that has some scientists rethinking humans' genetic destinies. Is our hereditary fate -- bipolar disorder or cancer at age 70, for example -- sealed upon the formation of our double helices, or are there things we can do to change it? Are we recipients of our DNA, or caretakers of it?