An in-depth study of the entire genomes of 128 Ashkenazi Jews has the potential to provide tools to prevent or treat genetic diseases common in the community. In the process, however, it has turned up evidence of an extraordinary genetic bottleneck 700 years ago.
Ashkenazi Jews, who until the 19th Century lived mostly in central and Eastern Europe, had a population of close to 16 million prior to the Holocaust. However, according to a study published in Nature Communications, between 25 and 32 generations ago there were just 350 ancestors.
This doesn’t mean the Ashkenazi population at that time was that small, but that any others from that era do not have living descendents. The small ancestral population helps explain why certain genetic conditions are common among Asheknazi Jews, including both some with disastrous effects when transmitted by both parents, but debated benefits when seen on only one chromosome.
The study found a fairly even mix of European and Middle-Eastern ancestry in the population, refuting the theory that Ashkenazi Jews have predominantly Khazar ancestry, a hypothesis supported by some credible researchers, but also often raised with racists motivations.
Co-author, Columbia University’s Itsik Pe’er says the study was done because research into genetic diseases in the Ashkenazi population had to use non-Jews as controls. When studying Ashkenazi genomes researchers would come across variations which did not appear in other populations, but Pe’er says “the genome was not there to distinguish the variants that are there and to tell us whether they are normal or whether we should get worried.”
The information obtained from those in the sample is already helping doctors identify variations that are not linked to disease so they can hone in on those that are, with the authors claiming it is "eightfold more effective at filtering benign variants" than comparisons with European populations as a whole.
Genetically isolated populations provide a sample that can be homogenous enough to study in depth, providing insights that can be applicable to the human population as a whole.
In addition to the insight into the history of Judism in over the last millennium, the study also found evidence that the European and Middle Eastern ancestors of Ashkenazi Jews had previously separated genetically 12 to 25,000 years ago.
“This is, I think, the first evidence from whole human genomes that the most important wave of settlement from the Near East was most likely shortly after the Last Glacial Maximum . . . and, notably, before the Neolithic transition—which is what researchers working on mitochondrial DNA have been arguing for some years,” Martin Richards of the University of Huddersfiled told The Scientist.