There are a lot of people who live around the coast plain of France, Spain, Portugal, and regions of the Med....who believe their ancestor path is 'pure'. Year, DNA tests are being done, and there's this minor bit of shock because....they have this northwest Europe and Denmark DNA marker that shows up. Why?
This goes to a historical situation, between the 790 AD era to the mid 1100 era....the period of Vikings and conquest.
If the Vikings had ever been united.....most of Europe would have fallen and become some 'semi-giant' Viking-land. But they weren't a unified type force, and their whole strategy was attack and rob. You'd have a few vessels pull up....some Vikings jump off, and walk a few miles inland, then stage an assault on some village....where rape was common.
Nine months later, a number of these women would produce kids, who were 50-percent Viking. Twenty-years later, the same Viking crowd might return....follow the same routine, and the 50-Viking woman would now produce a 75-percent Viking child.
So as much as people would be shocked to find a Viking 'marker' in their DNA....it's hard to avoid this unless you were Asian or Middle-Eastern.