CHICAGO—Maybe you CAN blame being overweight on your genes. But there’s a way to overcome that family history—just get three to four hours of moderate activity a day.

Sound pretty daunting?

Not for the Amish of Lancaster County, Pa., who were the focus of a new study on a common genetic variation that makes people more likely to gain weight. It turns out the variant’s effects can be blocked with physical activity—lots of it.

Scientists believe about 30 percent of white people of European ancestry have this variant, including the Amish, and that may partly explain why so many people are overweight.