Ashkenazi Jews all come from a founding population of about 350 people.

Ashkenazi Jews (AJ), identified as Jewish individuals of Central- and Eastern European ancestry, form the largest genetic isolate in the United States. AJ demonstrate distinctive genetic characteristics including high prevalence of autosomal recessive diseases and relatively high frequency of alleles that confer a strong risk of common diseases, such as Parkinson’s disease and breast and ovarian cancer. 

Several recent studies have employed common polymorphisms to characterize AJ as a genetically distinct population, close to other Jewish populations as well as to present-day Middle Eastern and European populations. Previous analyses of recent AJ history highlighted a narrow population bottleneck of only hundreds of individuals in late medieval times, followed by rapid expansion.

A new study improves imputation accuracy for AJ SNP arrays by 28%, and covers at least one haplotype in ≈67% of any AJ genome with long, identical-by-descent segments. Reconstruction of recent AJ history from such segments confirms a recent bottleneck of merely ≈350 individuals.

Modelling of ancient histories for AJ and European populations using their joint allele frequency spectrum determines AJ to be an even admixture of European and likely Middle Eastern origins. We date the split between the two ancestral populations to ≈12–25 Kyr, suggesting a predominantly Near Eastern source for the repopulation of Europe after the Last Glacial Maximum.

The study, in other words, found that 

“the central and eastern European Jewish population, known as Ashkenazi Jews, from whom most American Jews are descended, started from a founding population of about 350 people between 600 and 800 years ago.”

According to Columbia University researcher Itsik Pe’er, who was involved with the study, their research also showed that the group of 350 was made up of Jews of Middle Eastern and European—thereby disproving the much-debated theory that Jews descended from Khazars, a Turkic people who lived in the Caucasus region between the 7th and 10th centuries.