Efforts to identify the origins of Ashkenazi Jews through DNA analysis began in the 1990s. Currently, there are three types of genetic origin testing, autosomal DNA (atDNA), mitochondrial DNA (mtDNA), and Y-chromosomal DNA (Y-DNA). Autosomal DNA is a mixture from an individual's entire ancestry, Y-DNA shows a male's lineage only along his strict paternal line, mtDNA shows any person's lineage only along the strict maternal line. Genome-wide association studies have also been employed to yield findings relevant to genetic origins.
Like most DNA studies of human migration patterns, the earliest studies on Ashkenazi Jews focused on the Y-DNA and mtDNA segments of the human genome. Both segments are unaffected by recombination (except for the ends of the Y chromosome – the pseudoautosomal regions known as PAR1 and PAR2), thus allowing tracing of direct maternal and paternal lineages.
These studies revealed that Ashkenazi Jews originate from an ancient (2000 BCE – 700 BCE) population of the Middle East who had spread to Europe.
Ashkenazic Jews display the homogeneity of a genetic bottleneck, meaning they descend from a larger population whose numbers were greatly reduced but recovered through a few founding individuals.
Although the Jewish people, in general, were present across a wide geographical area as described, genetic research done by Gil Atzmon of the Longevity Genes Project at Albert Einstein College of Medicine suggests "that Ashkenazim branched off from other Jews around the time of the destruction of the First Temple, 2,500 years ago ... flourished during the Roman Empire but then went through a 'severe bottleneck' as they dispersed, reducing a population of several million to just 400 families who left Northern Italy around the year 1000 for Central and eventually Eastern Europe."
Various studies have arrived at diverging conclusions regarding both the degree and the sources of the non-Levantine admixture in Ashkenazim, particularly with respect to the extent of the non-Levantine genetic origin observed in Ashkenazi maternal lineages, which is in contrast to the predominant Levantine genetic origin observed in Ashkenazi paternal lineages. All studies nevertheless agree that genetic overlap with the Fertile Crescent exists in both lineages, albeit at differing rates. Collectively, Ashkenazi Jews are less genetically diverse than other Jewish ethnic divisions, due to their genetic bottleneck.
Male lineages: Y-chromosomal DNA
The majority of genetic findings to date concerning Ashkenazi Jews conclude that the male line was founded by ancestors from the Middle East.
A study of haplotypes of the Y-chromosome, published in 2000, addressed the paternal origins of Ashkenazi Jews. Hammer et al. found that the Y-chromosome of Ashkenazi and Sephardic Jews contained mutations that are also common among other Middle Eastern peoples, but uncommon in the autochthonous European population. This suggested that the male ancestors of the Ashkenazi Jews could be traced mostly to the Middle East. The proportion of male genetic admixture in Ashkenazi Jews amounts to less than 0.5% per generation over an estimated 80 generations, with "relatively minor contribution of European Y chromosomes to the Ashkenazim," and a total admixture estimate "very similar to Motulsky's average estimate of 12.5%." This supported the finding that "Diaspora Jews from Europe, Northwest Africa, and the Near East resemble each other more closely than they resemble their non-Jewish neighbors." "Past research found that 50–80 percent of DNA from the Ashkenazi Y chromosome, which is used to trace the male lineage, originated in the Near East," Richards said.
The population has subsequently spread out.
A 2001 study by Nebel et al. showed that both Ashkenazi and Sephardic Jewish populations share the same overall paternal Near Eastern ancestries. In comparison with data available from other relevant populations in the region, Jews were found to be more closely related to groups in the north of the Fertile Crescent. The authors also report on Eu 19 (R1a) chromosomes, which are very frequent in Central and Eastern Europeans (54–60%) at elevated frequency (13%) in Ashkenazi Jews. They hypothesized that the differences among Ashkenazim Jews could reflect low-level gene flow from surrounding European populations or genetic drift during isolation. A later 2005 study by Nebel et al., found a similar level of 11.5% of male Ashkenazim belonging to R1a1a (M17+), the dominant Y-chromosome haplogroup in Central and Eastern Europeans. However, a 2017 study, concentrating on the Ashkenazi Levites where the proportion reaches 50%, while signalling that there's a "rich variation of haplogroup R1a outside of Europe which is phylogenetically separate from the typically European R1a branches", precises that the particular R1a-Y2619 sub-clade testifies for a local origin, and that the "Middle Eastern origin of the Ashkenazi Levite lineage based on what was previously a relatively limited number of reported samples, can now be considered firmly validated."
Female lineages: Mitochondrial DNA
Before 2006, geneticists had largely attributed the ethnogenesis of most of the world's Jewish populations, including Ashkenazi Jews, to Israelite Jewish male migrants from the Middle East and "the women from each local population whom they took as wives and converted to Judaism." Thus, in 2002, in line with this model of origin, David Goldstein, now of Duke University, reported that unlike male Ashkenazi lineages, the female lineages in Ashkenazi Jewish communities "did not seem to be Middle Eastern", and that each community had its own genetic pattern and even that "in some cases the mitochondrial DNA was closely related to that of the host community." In his view, this suggested, "that Jewish men had arrived from the Middle East, taken wives from the host population and converted them to Judaism, after which there was no further intermarriage with non-Jews."
In 2006, a study by Behar et al., based on what was at that time high-resolution analysis of haplogroup K (mtDNA), suggested that about 40% of the current Ashkenazi population is descended matrilineally from just four women, or "founder lineages", that were "likely from a Hebrew/Levantine mtDNA pool" originating in the Middle East in the 1st and 2nd centuries CE. Additionally, Behar et al. suggested that the rest of Ashkenazi mtDNA is originated from ~150 women, and that most of those were also likely of Middle Eastern origin. In reference specifically to Haplogroup K, they suggested that although it is common throughout western Eurasia, "the observed global pattern of distribution renders very unlikely the possibility that the four aforementioned founder lineages entered the Ashkenazi mtDNA pool via gene flow from a European host population".
In 2013, a study of Ashkenazi mitochondrial DNA by a team led by Martin B. Richards of the University of Huddersfield in England reached different conclusions, in line with the pre-2006 origin hypothesis. Testing was performed on the full 16,600 DNA units composing mitochondrial DNA (the 2006 Behar study had only tested 1,000 units) in all their subjects, and the study found that the four main female Ashkenazi founders had descent lines that were established in Europe 10,000 to 20,000 years in the past while most of the remaining minor founders also have a deep European ancestry. The study argued that the great majority of Ashkenazi maternal lineages were not brought from the Near East or the Caucasus, but instead assimilated within Europe, primarily of Italian and Old French origins. The Richards study estimated that more than 80 percent of Ashkenazi maternal ancestry comes from women indigenous to Europe, and only 8 percent from the Near East, while the origin of the remainder is undetermined. According to the study these findings "point to a significant role for the conversion of women in the formation of Ashkenazi communities."
Karl Skorecki criticized the study for perceived flaws in phylogenetic analysis. "While Costa et al have re-opened the question of the maternal origins of Ashkenazi Jewry, the phylogenetic analysis in the manuscript does not 'settle' the question."
A 2014 study by Fernández et al. found that Ashkenazi Jews display a frequency of haplogroup K in their maternal DNA, suggesting an ancient Near Eastern matrilineal origin, similar to the results of the Behar study in 2006. Fernández noted that this observation clearly contradicts the results of the 2013 study led by Richards that suggested a European source for 3 exclusively Ashkenazi K lineages.
Association and linkage studies
In genetic epidemiology, a genome-wide association study (GWA study, or GWAS) is an examination of all or most of the genes (the genome) of different individuals of a particular species to see how much the genes vary from individual to individual. These techniques were originally designed for epidemiological uses, to identify genetic associations with observable traits.
A 2006 study by Seldin et al. used over five thousand autosomal SNPs to demonstrate European genetic substructure. The results showed "a consistent and reproducible distinction between 'northern' and 'southern' European population groups". Most northern, central, and eastern Europeans (Finns, Swedes, English, Irish, Germans, and Ukrainians) showed >90% in the "northern" population group, while most individual participants with southern European ancestry (Italians, Greeks, Portuguese, Spaniards) showed >85% in the "southern" group. Both Ashkenazi Jews as well as Sephardic Jews showed >85% membership in the "southern" group. Referring to the Jews clustering with southern Europeans, the authors state the results were "consistent with a later Mediterranean origin of these ethnic groups".
A 2007 study by Bauchet et al. found that Ashkenazi Jews were most closely clustered with Arabic North African populations when compared to Global population, and in the European structure analysis, they share similarities only with Greeks and Southern Italians, reflecting their east Mediterranean origins.
A 2010 study on Jewish ancestry by Atzmon-Ostrer et al. stated "Two major groups were identified by principal component, phylogenetic, and identity by descent (IBD) analysis: Middle Eastern Jews and European/Syrian Jews. The IBD segment sharing and the proximity of European Jews to each other and to southern European populations suggested similar origins for European Jewry and refuted large-scale genetic contributions of Central and Eastern European and Slavic populations to the formation of Ashkenazi Jewry", as both groups – the Middle Eastern Jews and European/Syrian Jews – shared common ancestors in the Middle East about 2500 years ago. The study examines genetic markers spread across the entire genome and shows that the Jewish groups (Ashkenazi and non Ashkenazi) share large swaths of DNA, indicating close relationships and that each of the Jewish groups in the study (Iranian, Iraqi, Syrian, Italian, Turkish, Greek and Ashkenazi) has its own genetic signature but is more closely related to the other Jewish groups than to their fellow non-Jewish countrymen. Atzmon's team found that the SNP markers in genetic segments of 3 million DNA letters or longer were 10 times more likely to be identical among Jews than non-Jews. Results of the analysis also tally with biblical accounts of the fate of the Jews. The study also found that with respect to non-Jewish European groups, the population most closely related to Ashkenazi Jews are modern-day Italians. The study speculated that the genetic-similarity between Ashkenazi Jews and Italians may be due to inter-marriage and conversions in the time of the Roman Empire. It was also found that any two Ashkenazi Jewish participants in the study shared about as much DNA as fourth or fifth cousins.
A 2010 study by Bray et al., using SNP microarray techniques and linkage analysis found that when assuming Druze and Palestinian Arab populations to represent the reference to world Jewry ancestor genome, between 35 and 55 percent of the modern Ashkenazi genome can possibly be of European origin, and that European "admixture is considerably higher than previous estimates by studies that used the Y chromosome" with this reference point. Assuming this reference point the linkage disequilibrium in the Ashkenazi Jewish population was interpreted as "matches signs of interbreeding or 'admixture' between Middle Eastern and European populations". On the Bray et al. tree, Ashkenazi Jews were found to be a genetically more divergent population than Russians, Orcadians, French, Basques, Sardinians, Italians and Tuscans. The study also observed that Ashkenazim are more diverse than their Middle Eastern relatives, which was counterintuitive because Ashkenazim are supposed to be a subset, not a superset, of their assumed geographical source population. Bray et al. therefore postulate that these results reflect not the population antiquity but a history of mixing between genetically distinct populations in Europe. However, it's possible that the relaxation of marriage prescription in the ancestors of Ashkenazim that drove their heterozygosity up, while the maintenance of the FBD rule in native Middle Easterners have been keeping their heterozygosity values in check. Ashkenazim distinctiveness as found in the Bray et al. study, therefore, may come from their ethnic endogamy (ethnic inbreeding), which allowed them to "mine" their ancestral gene pool in the context of relative reproductive isolation from European neighbors, and not from clan endogamy (clan inbreeding). Consequently, their higher diversity compared to Middle Easterners stems from the latter's marriage practices, not necessarily from the former's admixture with Europeans.
The genome-wide genetic study carried out in 2010 by Behar et al. examined the genetic relationships among all major Jewish groups, including Ashkenazim, as well as the genetic relationship between these Jewish groups and non-Jewish ethnic populations. The study found that contemporary Jews (excluding Indian and Ethiopian Jews) have a close genetic relationship with people from the Levant. The authors explained that "the most parsimonious explanation for these observations is a common genetic origin, which is consistent with an historical formulation of the Jewish people as descending from ancient Hebrew and Israelite residents of the Levant".
The Khazar hypothesis
In the late 19th century, it was proposed that the core of today's Ashkenazi Jewry are genetically descended from a hypothetical Khazarian Jewish diaspora who had migrated westward from modern Russia and Ukraine into modern France and Germany (as opposed to the currently held theory that Jews migrated from France and Germany into Eastern Europe). The hypothesis is not corroborated by historical sources and is unsubstantiated by genetics, but it is still occasionally supported by scholars who have had some success in keeping the theory in the academic consciousness.
The theory has sometimes been used by Jewish authors such as Arthur Koestler as part of an argument against traditional forms of antisemitism (for example the claim that "the Jews killed Christ"), just as similar arguments have been advanced on behalf of the Crimean Karaites. Today, however, the theory is more often associated with antisemitism and anti-Zionism.
A 2013 trans-genome study carried out by 30 geneticists, from 13 universities and academies, from 9 countries, assembling the largest data set available to date, for assessment of Ashkenazi Jewish genetic origins found no evidence of Khazar origin among Ashkenazi Jews. "Thus, analysis of Ashkenazi Jews together with a large sample from the region of the Khazar Khaganate corroborates the earlier results that Ashkenazi Jews derive their ancestry primarily from populations of the Middle East and Europe, that they possess considerable shared ancestry with other Jewish populations, and that there is no indication of a significant genetic contribution either from within or from north of the Caucasus region", the authors concluded.
There are many references to Ashkenazi Jews in the literature of medical and population genetics. Indeed, much awareness of "Ashkenazi Jews" as an ethnic group or category stems from the large number of genetic studies of disease, including many that are well reported in the media, that have been conducted among Jews. Jewish populations have been studied more thoroughly than most other human populations, for a variety of reasons:
- Jewish populations, and particularly the large Ashkenazi Jewish population, are ideal for such research studies, because they exhibit a high degree of endogamy, yet they are sizable.
- Jewish communities are comparatively well informed about genetics research, and have been supportive of community efforts to study and prevent genetic diseases.
The result is a form of ascertainment bias. This has sometimes created an impression that Jews are more susceptible to genetic disease than other populations. Healthcare professionals are often taught to consider those of Ashkenazi descent to be at increased risk for colon cancer.
Genetic counseling and genetic testing are often undertaken by couples where both partners are of Ashkenazi ancestry. Some organizations, most notably Dor Yeshorim, organize screening programs to prevent homozygosity for the genes that cause related diseases.