Jewish I is described as "the most common of the three markers . . . its frequency highest in Poles, Russians, Germans, Hungarians, Romanians and Slavic peoples who intermarried with Ashkenazi Jews. It also appears in Italian, Spanish, Portuguese and Moroccan Jews (Sephardim)." This means it is a marker of general European admixture. But where is it most common, and where did the European Jews it identifies originate? Where were they concentrated in earliest times?
Statistical searches in ENFSI provide some tantalizing answers. Jewish I is found at a relatively high frequency throughout Europe but at elevated levels in Poland, Portugal and Belgium. It is lowest in Italy and Finland.
This would suggest that Jewish ancestry and intermarriage was common in countries where the population still carries this admixture marker.