|
Ashkenazi Jews According to the scriptures, the Jewish people originated from Semitic tribes that lived in the Middle East approximately 4,000 years ago. In 587 B.C.E., after the defeat of the Judean kingdom, the Jews were dispersed and exiled into Babylonia and other areas. Recent population studies using genetic markers, however, support the common Middle Eastern origin of the Jews.
Ashkenazi Jews, that is, those Jews of Eastern European origin, constitute more than 80 percent of all world Jewry. The early founders of the Ashkenazi community made their way to Europe during Roman rule, but the majority of the founders of the population came more recently from the region of present day Israel, moved to Spain, France, and Italy, and then in the 10th century into the Rhineland valley in Germany. It is estimated that prior to 1096, the first Crusade, the entire Jewish population of Germany comprised 20,000 people.
During the Crusades The Crusades, carried out between the 11th and 13th centuries, and the charters of protection granted to the Jews in 1264, in order to rebuild Polish cities, encouraged the migration of many Jews to Poland and Lithuania. In 1334 the charter safeguarding Jewish liberties was extended to all Polish duchies that were united into one kingdom. The early Jewish settlements in these areas often consisted of only a few families that remained isolated genetically, not only from their non-Jewish neighbors, but also from other Jewish communities. Until the end of the Middle Ages, the Jewish population of Eastern Europe was very small. It is estimated that in 1300 there were only about 5,000 Jews in all of Poland and Lithuania; by 1490 their number had increased to between 10,000 and 30,000. In the mid-1600s the Cossack uprising led to the massacre of many of the Jews of Poland and Ukraine.
The 18th and 19th Centuries In the 18th century there was a significant growth in the number of Ashkenazi Jews, which, based on the 1765 census, reached 750,000 in number. At the end of the following century, 6 million Jews were counted in an additional census. Persecutions and economic difficulties in the 19th century caused many Ashkenazi Jews to emigrate to the United States and to Israel.
Recent History During the Holocaust 6 million European Jews perished. After World War II many of the survivors immigrated to Israel. In the last 10 years 800,000 Russian Jews have immigrated from the former U.S.S.R to Israel. The present-day Ashkenazi Jewish population is estimated at more than 10 million, of which 2.8 million live in Israel.
A Unique (Genetic) Population The Ashkenazi Jewish population is a unique population in terms of its demographic history and genetic architecture. The singular demographic history of the Jewish Ashkenazi population includes emergence from a limited number of founders, exceptional expansion and contraction of the population size and a long history of marriage within the community and the faith (endogamy). In addition, the unequal reproduction rate between the social classes of the Ashkenazi Jews made the effective population size much smaller. This means that the current Ashkenazi population did not originate from all of the founders living many centuries ago, but selectively from a small fraction of wealthier people.
The consequence of the unique demographic history of the Ashkenazi Jews is that they have a more homogeneous genetic background compared to the general population.
There are several rare genetic diseases, which occur with a particularly high incidence among Ashkenazi Jews, including Tay-Sachs, Gaucher disease, Bloom syndrome, Idiopathic torsion dystonia, Familial dysautonomia, Factor XI deficiency, and more. For many of these disorders in which a causative gene has been identified, a specific mutation was found to be the cause of most cases of the disease in Ashkenazi Jews.
The frequencies of the common mutations of most of these diseases are between 1/16 and 1/110 among Ashkenazi Jews. Each of these mutations is found mainly in the context of a single haplotype, a finding consistent with a single founder of each mutation. For example, the gene causing Familial dysautonomia (FD) was mapped to a segment of chromosome 9 using Ashkenazi families. The candidate region was confirmed by haplotype analysis; haplotype sharing among 435 out of 441 FD chromosomes revealed a strong founder effect.
Age estimations were calculated by a genetic clock for most of the high frequency diseases listed above. The origin of the investigated mutations can be all dated to between the 9th and 14th centuries and is consistent with the early migration of Jews to Europe and the founding of the Jewish community in Eastern Europe. The high frequency of some genetic disorders among the Ashkenazi Jews (less than 1/100) indicates that the founder chromosome carrying the disease allele was introduced into a very small population, probably in the order of about 100 unrelated individuals. The demographic expansion that followed the early migration maintained the high frequency of these alleles and was probably also affected by genetic drift.
References and Suggested Reading Behar, D.M. (2006). The matrilineal ancestry of Ashkenazi Jewry: portrait of a recent founder event. American Journal of Human Genetics 78:487-97.
Goodman, R.M. (1979). Genetic diseases among Ashkenazi Jews. New York: Raven Press. pp. xiii, 440.
Motulsky, A.G. (1995). Jewish diseases and origins. Natural Genetics 9:99-101.
Zlotogora, J. (1994). High frequencies of human genetic diseases: founder effect with genetic drift or selection? American Journal of Medical Genetics 49:10-3.
|
