[who_would_fardels_bear]

The story I heard is that the European Jews got there primarily by working their way up the Italian peninsula over generations until they settled in places that ultimately became Poland, Germany, etc.

If they were a mostly insular group, either by choice or forced segregation, then they would have retained their unique genetic markers. However, if there was some intermarriage with local populations then those markers would be significantly diluted with some self-proclaimed Jews having no Jewish genetic markers.

[Angelino97]

There was a lot of intermarriage in Spain during the Muslim occupation, especially among the ruling elite. Intermarriage among Jewish, Christian, and Muslim nobles, merchants, doctors (i.e., the upper classes) was not uncommon.

I've read that 40% to 45% of Spaniards have traces of Sephardic Jewish DNA.

[jjotto]

Ashkenazi DNA (15 minutes)

https://youtu.be/89D2RDgzLLE?si=XN_g-F3UPRrZzRhx

[MeanWestTexan]

The closet ancestors genetically to Ashkenazi Jews are Sephardic Jews who never left the Levant, followed by the Samaritans who never left the Levant , followed by Arabs in and around Israel.

It’s well studied and no mystery.