Among scientists who study how our DNA affects our weight, a gene called FTO stands out. “It’s the poster child for the genetics of obesity,” said Struan F. Grant, an associate professor of pediatrics at the University of Pennsylvania School of Medicine.

In 2007, researchers discovered that people with a common variant of FTO tend to be heavier than those without it. Since then, studies have repeatedly confirmed the link. On average, one copy of the risky variant adds up to 3.5 extra pounds of weight. Two copies of the gene bring 7 extra pounds — and increase a person’s risk of becoming obese by 50 percent.

But the gene doesn’t seem to have always been a problem. If scientists had studied FTO just a few decades ago, they would have found no link to weight whatsoever. A new study shows that FTO became a risk only in people born after World War II.

The research, published this week in the Proceedings of the National Academy of Sciences, raises questions that extend far beyond obesity. Genes clearly influence our health in many ways, but so does our environment; often, it is the interplay between them that makes the difference in whether we develop obesity or cancer or another ailment.