Ashkenazi Jews are one of the major ancestral groups of the Jewish people, encompassing those whose ancestors most recently lived in France, Central Europe, and Eastern Europe. The name originates from Ashkenaz, the Hebrew name for the area along the Rhine where diaspora Jews settled during the Middle Ages, before they moved eastward. By the global Jewish population’s peak, right before WWII, Ashkenazim had grown to encompass around 90% of Jews; today, about two-thirds of the world’s Jews are Ashkenazi. However, Ashkenazim and other Jewish ancestral groups are not evenly distributed across the globe. In the US, most Jews are Ashkenazi, but in Israel, only around 40% of the Jewish population is Ashkenazi (sources vary from 32% to 45%, potentially due to mixing between Jewish ethnic groups).
Ashkenazi DNA has been heavily studied, mostly due to the prevalence of genetic diseases, but scientists have yet to uncover the whole story of Ashkenazi migration and development into a separate Jewish ethnic group. The genetic makeup of Ashkenazi Jews is particularly noteworthy because of its homogeneity in comparison to other ethnic groups. In other words, Ashkenazim have an unusually high degree of genetic similarity. Scientists believe this genetic similarity to be a result of a bottleneck or founder event, in which a new population is created by a very small number of individuals from an originally larger population. Since the new population starts out with fewer individuals, and therefore fewer distinct genomes, a loss of genetic variation occurs.
Two (relatively) recent studies highlight the potential of a bottleneck effect in the Ashkenazi population: Behar et al. (2004) and Behar et al. (2006). Both studies involve the use of haplogroups (sets of genetic markers that are inherited from a single parent), which are particularly sensitive to the genetic drift of a population. The 2004 study focuses on Y chromosome variation, inherited paternally, in Ashkenazi Jews compared to non-Jewish European populations. The researchers found that seven Y chromosome haplogroups accounted for 84.5% of the Ashkenazi Jewish chromosomes studied. The lineages and prevalence of these haplogroups present evidence for low genetic variability, probably caused by a bottleneck effect. Additionally, four of these seven main haplogroups were likely part of an ancestral gene pool from Jews who had migrated from the Middle East, and the researchers identified an additional three Ashkenazi haplogroups that were presumably picked up in Europe. An additional study based on Behar et al.’s (2004) results found that there was minimal admixture of European genes into the Ashkenazi population after its original foundation around 600 years ago, which is interesting in terms of Ashkenazi culture and the limited intermarriage with surrounding European populations.
The 2006 study analyzes the mitochondrial genomes, which are inherited maternally, of Ashkenazi Jews. The researchers discovered that four mitochondrial haplotypes accounted for more than 40% of the studied Ashkenazi population. These results suggest the existence of four main founder females. The study also determined that these women were likely of Middle Eastern ancestry. Other Jewish populations, like the Jews of Tunisia and Cochin (South India), also exhibit founder effects, but the Ashkenazi founder effect is unique in that there is such a large current population (more than 10 million) for relatively few founders.
Many such studies examine Ashkenazi genetic history in terms of a population bottleneck: a steep decrease in population size due to environmental or human factors. Bottlenecks are usually caused by catastrophes, massacres, and/or discrimination that prevents marriage outside of the community. As all of these options are common throughout Jewish history, it is currently unknown which specific events caused the population bottleneck from which the modern Ashkenazi population stems.
However, recent studies have helped pin the timing of this bottleneck. A 2022 study describing the genetic makeup of 14th-century Jews reveals that the Ashkenazi bottleneck happened earlier than previously thought. Before, scientists had theorized that the bottleneck occurred in the 13th and 14th centuries with the start of the Crusades and the rise in antisemitism as a result of the Black Death. In the study, a Jewish cemetery was accidentally uncovered under a granary in Erfurt, Germany. In order to sample the ancient DNA, scientists had to work around Jewish law that demands the dead remain undisturbed. With the advice of a rabbi, the researchers collected the stray teeth of 38 individuals from the cemetery (teeth fall out naturally, so it was deemed “kosher” to sample them). These teeth contained disease mutations and long stretches of identical code similar to those found in today’s Ashkenazi population. Therefore, the bottleneck in question must have occurred well before the lifetime of these Jews. An additional study of Ashkenazi remains from 1190 in Norwich, Germany confirmed these results. These studies verify the theory that the modern Ashkenazi population stems from a few hundred people who survived a bottleneck in Europe at least 1,000 years ago.
Unfortunately, the low genetic variability caused by a bottleneck effect means that today, Ashkenazi Jews have high rates of certain genetic diseases because many people in the population carry identical mutations. The molecular basis of more than 25 genetic diseases has been identified in Ashkenazi populations, mostly as a result of one or two founder mutations present in elevated frequencies. Ashkenazi Jews are at a higher risk for genetic diseases such as Tay-Sachs (an inherited condition that is fatal in babies), Gaucher, Parkinson’s, and Crohn’s. They are also more prone to blood clotting deficiencies, steroid hormone defects, and defects in DNA repair that cause an increased susceptibility to cancer, especially breast and ovarian cancers. As a result, genetic screening is prevalent among Ashkenazim for reproductive risks, identification of disease risks, and treatment of genetic disease. For example, screening for Tay-Sachs for Ashkenazi Jews has been in practice since the 70s; its implementation significantly reduced the number of Tay-Sachs births per year. Genetic screening for three specific cancer-causing mutations in Ashkenazim has helped prevent cancers through preemptive mastectomies and oophorectomies (removal of the ovaries). Knowledge of the genetic defect that can cause Gaucher disease has actually led to the creation of a very effective enzyme-replacement therapy. Today, Ashkenazi Jews are typically screened for 18 conditions. As such, it is unsurprising that many medical forms include a checkbox for “Ashkenazi Ancestry.”
As much as Ashkenazi DNA is interesting from a scientific perspective, it is also the subject of debate as it relates to the ethnic and racial identity of Ashkenazim. As highlighted in the studies previously mentioned, Ashkenazi DNA is a mixture of Middle Eastern genomes from Jews who migrated to Europe (many as a result of exile, persecution, or even slavery) and European genomes picked up as they moved throughout the continent. Various amounts of European genomic material (30-60%) have been found in Ashkenazi, Sephardic, Italian, and Syrian Jews. Ashkenazi Jews are not genetically fully European, just as they are not (and were not, even when they lived in Europe) fully culturally, linguistically, or politically European. To label Ashkenazi Jews as simply “Europeans” erases these distinctions between the Ashkenazim and their host populations, the intense discrimination they faced for not being “European enough,” their connection to their ancestral homeland, and their ties to other Jewish ethnic groups.
Just as the addition of some European genes into the Ashkenazi population does not negate their historical, genetic, and anthropological indigeneity to the Levant¹ and connection to a culture that originated in the Middle East, neither does the fact that some Ashkenazim are white/white-passing. Even though Ashkenazim are largely considered white in an American racial context, they have not been seen as such in the past and in other parts of the world. (Not to mention, many other Middle Eastern peoples, especially those from the Levant, can also pass as white.) Of course, this does not discredit the fact that white/white-passing Jews do benefit from white privilege today and have benefited from it in the past.
To conclude, the genetic makeup of Ashkenazim in particular yields scientific interest since the population has very limited genetic diversity, which is likely the result of a bottleneck effect. Unfortunately, this reduction in genome variability results in the tendency for Ashkenazi Jews to be especially susceptible to genetic diseases. Although Ashkenazi ethnicity is sometimes debated in a political context, scientifically, Ashkenazi Jews and other Jewish diaspora populations have a major Middle East (Levantine) ancestry that is combined with differing degrees of genetics from host diaspora populations.
[1] The Levant refers to the coastal region of the Middle East that encompasses modern-day Syria, Lebanon, Israel, Palestine, Jordan, and Cyprus. Sometimes parts of Egypt, Greece, and Turkey are considered to be included in the Levant.