By ethnicity
In an ethnic sense, an Ashkenazi Jew is one whose ancestry can be traced to the Jews of Central and Eastern Europe. For roughly a thousand years, the Ashkenazim were a reproductively isolated population in Europe, despite living in many countries, with little inflow or outflow from migration, conversion, or intermarriage with other groups, including other Jews. Human geneticists have identified genetic variations that have high frequencies among Ashkenazi Jews, but not in the general European population. This is true for patrilineal markers (Y-chromosome The Y chromosome is the sex-determining chromosome in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development, thus determining sex. The human Y chromosome is composed of about 60 million base pairs haplotypes The term haplotype is a contraction of the term 'haploid genotype'. In genetics, a haplotype is a combination of alleles at multiple loci that are transmitted together on the same chromosome. Haplotype may refer to as few as two loci or to an entire chromosome depending on the number of recombination events that have occurred between a given set) as well as for matrilineal markers (mitochondrial In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers (μm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical haplotypes).[11]
Since the middle of the 20th century, many Ashkenazi Jews have intermarried, both with members of other Jewish communities and with people of other nations and faiths, while some Jews have also adopted children from other ethnic groups or parts of the world and raised them as Jews. Conversion to Judaism, rare for nearly 2,000 years, has become more common. Jewish women and families who choose artificial insemination often choose a biological father who is not Jewish, to avoid common autosomal recessive genetic diseases.
A study by Michael Seldin, a geneticist at the University of California Davis School of Medicine, found Ashkenazi Jews to be a clear, relatively homogenous genetic subgroup. Strikingly, regardless of the place of origin, Ashkenazi Jews can be grouped in the same genetic cohort — that is, regardless of whether an Ashkenazi Jew's ancestors came from Poland, Russia, Hungary, Lithuania, or any other place with a historical Jewish population, they belong to the same ethnic group. The research demonstrates the endogamy of the Jewish population in Europe and lends further credence to the idea of Ashkenazi Jews as an ethnic group. Moreover, though intermarriage among Jews of Ashkenazi descent has become increasingly more common, many Ultra-Orthodox Jews, particularly members Hasidic or Hareidi sects, continue to marry exclusively fellow Ashkenazi Jews. This trend keeps Ashkenazi genes prevalent and will also help researchers further study the genes of Ashkenazi Jews with relative ease. It is also noteworthy that these Ultra-Orthodox Jews often have extremely large families.[12]
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