Most heart attacks are caused by plaque embedded within the artery wall that ruptures, cracking the wall and triggering the formation of a blood clot. The clot blocks the flow of blood to the heart muscle, which can die from lack of oxygen and nutrients. Suddenly, the pump stops pumping.

Contrary to the clogged pipes model, heart attacks generally occur in arteries that have minimal or moderate blockage, and their occurrence depends more on the kind of plaque than on the quantity. Scientists have been struggling to figure out what type is most responsible. Paradoxically, findings suggest that immature, softer plaques rich in cholesterol are more unstable and likely to rupture than the hard, calcified, dense plaques that extensively narrow the artery channel. But understanding the root cause of the disease will require much more research. For one thing, human hearts, unlike plumbing fixtures, are not stamped from a mold. Like the rest of our body parts, they are products of our genes.

Don Steffensen was putting duck-hunting decoys out on a small lake one fall afternoon in southwestern Iowa when his heart attack hit. The infarction was massive and unexpected. It’s likely that Steffensen survived only because a buddy was carrying nitroglycerin tablets and quickly slipped one under his friend’s tongue. Nitroglycerin is used to make dynamite; in the body, a heavily diluted form releases nitric oxide, which signals the smooth muscle cells in veins and arteries to relax, dilating the vessels.

The Steffensen clan is enormous: more than 200 relatives spread over three generations, many of the youngest are now dispersed from Iowa to New York and beyond. Although heart trouble is common in the family, it had never struck anyone as unusual. “I attributed it to diet,” shrugs Tina, a slim 38-year-old and the family’s only vegetarian.

It was a reasonable conclusion. The Steffensens were raised on the kind of farm food that the state is famous for—ham balls, meatloaf, pie, macaroni and cheese—and still popular even as careers have moved indoors. Driving north through cornfields to meet some of the family in Buffalo Center, I dined at a restaurant offering deep-fried sandwiches. A single ham and cheese hoagie—dunked in hot fat and served sizzling—seemed capable of stopping a heart all on its own.

But could the high incidence of heart trouble among the Steffensens be related to something else besides high-fat diets? Eleven years after Don’s attack, his wife, Barbara, happened to overhear a doctor describing a study about the genetics of heart attacks.