“Jews really are different from their non-Jewish neighbors,” said Dr. Harry Ostrer, a geneticist at the New York University Langone Medical Center, coauthor of the study appearing Thursday in the American Journal of Human Genetics.

They are not different enough to be considered a separate race, as some experts have argued, he added, but definitely are a “distinct population” — the result, presumably, of cultural separation down through thousands of years.

The study, which was conducted primarily to further medical knowledge of genetic diseases, rejected a highly controversial idea that Ashkenazi Jews are descended from Khazars in Eastern Europe who converted to Judaism — an idea that has recently been used in an attempt to discredit the idea that Jews belong in Israel because it is their historic homeland.

Said Joe Berkofsky, a spokesman for the Jewish Federations of North America: “This finding in a way underscores what Jewish Federations believe and act upon through our central mission, which is to care for and protect Jews around the world, no matter where they are.”

Although the study sheds light on Jewish history — providing new information about the separation between North African and European Jews 2,500 years ago and the near extinction of European Jews in the Middle Ages — its major goal is to identify genes for many diseases that are more common in Jewish groups, such as breast cancer, Gaucher’s disease and Tay-Sachs.

The higher incidence of those diseases among “Abraham’s children” will allow scientists to more readily find genes that causes the illnesses and then extend that knowledge to the general population, said geneticist Gil Atzmon of Yeshiva University’s Albert Einstein College of Medicine in New York, coauthor of the paper.

The study examined 237 Jewish individuals from seven regions of the world, comparing them with 418 non-Jewish people from the same regions. Each of the Jewish subjects had all four grandparents from the same population.

The researchers studied about 160,000 sites across the entire genome, providing a great deal more information about the population than has ever been available.

Previous studies had found similar results by looking at smaller populations and considering only blood groups, mitochondrial DNA (a type of DNA passed down by mothers) or Y chromosomes (passed down by fathers).

The Jewish people, according to archaeologists, originated in Babylon and Persia between the 4th and 6th centuries BC. The modern-day Jews most closely related to that original population are those in Iran, Iraq and Syria, whose closest non-Jewish relatives are the Druze, Bedouins and Palestinians, the study found.

Sometime in that period, the Middle Eastern and European Jews diverged and the European branch began actively proselytizing for converts.

At the height of the Roman Empire, about 10% of the empire’s population was Jewish, although the bulk of them were converts. Some Khazars were also incorporated during this period.

“That explains why so many European and Syrian Jews have blue eyes and blond hair,” Ostrer says. It also explains another of the team’s findings — that the population most closely related genetically to European Jews are Italians.

The data also show what the researchers call a “bottleneck” in the Jewish population during the Middle Ages. The population of European Jews shrunk below 50,000 during that period because of disease, prejudice, anti-Semitic edicts and the Crusades, Atzmon said.

Afterward, however, an easing of restrictions led to what is known as the “demographic miracle,” in which the Jewish population rose twice as fast as that of other Europeans, reaching more than 5 million by the 19th century.

thomas.maugh@latimes.com