WASHINGTON — Scientists have pinpointed genetic variations that make people more likely to get hooked on cigarettes and more prone to develop lung cancer — a finding that could someday lead to screening tests and customized treatments for smokers trying to kick the habit.

The discovery makes the strongest case so far for the biological underpinnings of nicotine addiction and sheds more light on how genetics and lifestyle habits join forces to cause cancer.

“This is kind of a double whammy gene,” said Christopher Amos, a professor of epidemiology at the M.D. Anderson Cancer Center in Houston and author of one of the studies. “It also makes you more likely to be dependent on smoking and less likely to quit smoking.”

A smoker who inherits these genetic variations from both parents has an 80 percent greater chance of lung cancer than a smoker without the variants, the researchers reported. And that same smoker has a much harder time quitting than smokers who don’t have these genetic differences.

The three studies, funded by governments in the U.S. and Europe, are being published today in the journals Nature and Nature Genetics.

The scientists studied the genes of more than 35,000 white people of European descent in Europe, Canada and the United States. Blacks and Asians will be studied soon and may yield different results, scientists said.

They aren’t quite sure if what they found is a set of variations in one gene or in three closely connected genes.

The gene variations, which govern nicotine receptors on cells, could eventually help explain some of the mysteries of chain smoking, nicotine addiction and lung cancer. These oddities include why there are 90-year-old smokers who don’t get cancer and people who light up an occasional cigarette and don’t get hooked.

Among the findings:

Smokers who get the set of variants from only one parent see a risk of lung cancer that is about one-third higher than that of people without the variants.

Smokers who inherit the variants from both parents have nearly a 1-in-4 chance of developing lung cancer. Their cancer risk is 70 percent to 80 percent higher than that of smokers without the genetic variants.

Smokers who don’t have the variants are still more than 10 times more likely to get lung cancer than nonsmokers.

Smoking-related diseases worldwide kill about one in 10 adults, according to the World Health Organization.