Ashkenazi Jews hail from Eastern Europe—"Ashkenaz" is the Hebrew word for Germany—and comprise the bulk of the Jewish population in the US. Their compatriots are Sephardi Jews, who lived in Spain until they were kicked out in 1492, and Mizrachi Jews, who lived in Arab countries for centuries until the founding of the state of Israel in 1948.

Ashkenazi Jews are a genetically distinct population, and the analysis of 128 complete Ashkenazi genomes shows just how distinct they are. Compared to genomes of modern Europeans—in this case, the genomes of 26 Flemish people—the Ashkenazi genomes have 47 percent more novel DNA differences per genome. These DNA variants, while novel, are population specific; sequence sharing, where two individuals have a set of the same variants, is eight percent more abundant among the Ashkenazi Jews than it is between the two populations or even among the Flemish genomes.

Using the length of the shared genetic segments, researchers determined that the current Ashkenazi Jewish population underwent a bottleneck 25-32 generations back, approximately 600-800 years ago. Caused by the Plague in the mid 1300s, perhaps? Or maybe by the decimation of Eastern European Jewish communities by roving bands of Crusaders? Whatever the cause, this bottleneck reduced the founder population to between 250 and 420 individuals.

When populations undergo bottlenecks that reduce their founder sizes, rare and recessive mutations become amplified. That is exactly what happened to Ashkenazi Jews. Pregnant Ashkenazi women currently get screened for a panel of thirty-six recessive diseases, including Tay-Sachs disease, Canavan disease, Gaucher disease, Fanconi anemia, and cystic fibrosis.

They also have higher rates of BRCA1 and BRCA2 mutations, estimated to occur in 1 in 40 individuals compared to 1 in 400 in the general population. These mutations cause 11 percent of the breast cancer and 40 percent of the ovarian cancer in the Ashkenazi Jewish population.

Defining the extent of the unique Ashkenazi DNA variation, as this study has done, will make clinical diagnosis much simpler. Now, when an individual's genome is sequenced, it can be compared against this panel rather than one from the general European population, and doctors will more easily be able to ascertain which mutations might make a patient sick.

Nature Communications, 2014. DOI: 10.1038ncomms5835 (About DOIs).