Last week, researchers in the United States and France announced a surprising discovery: a single genetic variation is responsible for 18 to 37 percent of Parkinson's disease cases in Ashkenazi Jews and North African Arabs. While some unusual forms of Parkinson's are carried in families, scientists had thought genetics played only a minor role in most cases of the disease.

The discovery of the gene variation, which researchers think originated in a single person in the Middle East approximately 2,000 years ago, is important in understanding the causes of Parkinson's. It also reflects a growing tactic used by those hunting for the causes of diseases.

More and more scientists are studying populations of specific ancestry in order to hunt down genes linked to complex diseases, such as diabetes and asthma, as well as diseases such as Parkinson's, which were not thought to have a strong genetic component.

In the past, geneticists have searched for genes involved in a few rare diseases, such as Huntington's disease, in which a defect in a single gene causes the disorder. The technique works for these diseases because, in a small population with little genetic variation, it is relatively easy to identify the specific genetic difference that distinguishes people with the disease from those without it.

But finding genetic causes in more common illnesses caused by many genes interacting has been far harder. Now an explosion in genetic information -- led by better genotyping technologies that allow scientists to analyze thousands of different genetic markers in each individual -- is making it possible to study more complex diseases.

Population geneticists are particularly enthusiastic about studying the genetic signatures of two types of communities: so-called "founder" populations, such as Ashkenazi Jews and French Canadians, in which only a few ancestors contributed to the population; and admixture populations, in which two historically separate populations have mingled, such as African Americans, who have both African and European roots.

"If you're looking at a recently mixed population or a population with recent founder affects, the genome is a lot simpler," says David Reich, a geneticist at Harvard Medical School in Boston. "In these populations, the genome comes in big chunks of shared ancestry."