Italians Are Genetically Distinct from Jews

It's often claimed that Italians, especially Southern Italians, cluster with Ashkenazi Jews. At the very least, that's backwards because it's Jews who have mixed origins, pulling them away from West Asia and toward Europe. That, and the correspondence between geographic and genetic distance, puts them closest to Southern Europeans. However, when enough markers are used (or the right kinds of markers) the two groups can be distinguished, just as each can be distinguished from others. Indeed, Ashkenazi Jews form their own unique cluster, while Italians belong to a broader Southern European cluster, expectedly plotting between Spaniards and Greeks.

Tian et al. (2009) sampled Lombards (ITN_N), Tuscans (TUSC), Sardinians (SARD) and Southern Italian-Americans from New York (ITN), as well as Ashkenazi Jews (AJA), and genotyped them for 300,000 autosomal SNPs:

The current study extends the analysis of European population genetic structure to include additional southern European groups and Arab populations. Even within Italy, the relative position of northern Italians compared with subjects from Tuscany is consistent with the general geographic correspondence of PCA results. Interestingly, the majority of Italian Americans (NYCP four grandparents defined) appear to derive from southern Italy and overlap with subjects of Greek heritage. Both of these observations are consistent with previous historical information.

Possible exceptions to this observation of geographic correspondence include the Ashkenazi Jewish population. While the Ashkenazi are clearly of southern origin based on both PCA and STRUCTURE studies, in our analyses of diverse European populations, this group appears to have a unique genotypic pattern that may not reflect geographic origins.

Price et al. (2008) sampled Southern Italians and Sicilians, and isolated the genetic markers that are most accurate for distinguishing between European groups, achieving results comparable to those from genome-wide analyses:

Important work has already shown that northwest and southeast Europeans can be distinguished using as few as 800-1,200 ancestry-informative markers mined from datasets of 6,000-10,000 markers. Here we mine much larger datasets (more markers and more samples) to identify a panel of 300 highly ancestry-informative markers which accurately distinguish not just northwest and southeast European, but also Ashkenazi Jewish ancestry. [...] Our results are consistent with a previous study in which Ashkenazi Jewish and southeast European samples occupied similar positions on the northwest-southeast axis, although there was insufficient data in that study to separate these two populations.

Tian et al. (2008) provide an additional example of the same clustering pattern, using samples and markers similar to those in their other study:

European population genetic substructure was examined in a diverse set of >1,000 individuals of European descent, each genotyped with >300 K SNPs. Both STRUCTURE and principal component analyses (PCA) showed the largest division/principal component (PC) differentiated northern from southern European ancestry. A second PC further separated Italian, Spanish, and Greek individuals from those of Ashkenazi Jewish ancestry as well as distinguishing among northern European populations. In separate analyses of northern European participants other substructure relationships were discerned showing a west to east gradient.