Amid the holiday season with its boundless opportunities for piling on the pounds, recent findings about the genetics of obesity may give dieters a confusing message, according to weight control experts.

On one hand, enhanced understanding of how genes determine the shape and size of someone's body can help people be more realistic about what they can and should strive to look like. Some people were just not meant to be slender.On the other hand, overweight people can become adept at rationalizing.

Instead of trying to sensibly watch their weight, or be honest with themselves about precisely how many potato chips or chocolate cookies they are mindlessly nibbling in front of the television set, they might just blame it all on the genes.

"It could become the 1990s' equivalent of `It's my metabolism.' Now it could be `It's my genes,' " said Dr. Robert Kushner, a weight management specialist at the University of Chicago Medical School.

Experts have long known there is a genetic component to weight. But they also know genes are not the whole story.

Even if someone has a genetic predisposition to get chubby, it's a high-fat diet, a couch-potato lifestyle, or a tendency to eat for emotional reasons, rather than to satisfy genuine hunger, that turns the tendency into actual pounds.

"No one knows how much is genetic - estimates range from 20 to 70 percent," said Dr. Michael Hamilton of Duke University's well-known diet and fitness center. "But 20 to 70 percent leaves room for what you eat and your physical activity."

Lifestyle explains why Americans are getting heavier and heavier, even though they have the same genes.

"In 1980, about one in four Americans was overweight. Now the number is one in three," according to Bonnie Liebman, a nutritionist at the Center for Science in the Public Interest.

"That's a large difference - but it wasn't our gene pool that changed since 1980," Liebman said, noting that there is so little physical activity in modern society that "we don't even roll down our own car windows anymore."

Breakthroughs in genetics, like the discovery announced this month of a gene that helps the body regulate its fat content, do hold promise of devising new ways of treating obesity eventually.

But that's still off in the future, and obese people should not count on an easy "cure" that will take the place of diet, exercise and modified behavior.