The invading Romans did not displace the pre-existing population of Gauls in what is now modern France. The invading Franks did not displace the pre-existing population of Roman Gaul. The people who lived in France in 1600 were mostly descended from the people who lived there in 1600 BCE.
The people of modern Italy and France are descended from an ancient colonization of Western Europe. The invading populations of Goths and Lombards were able to usurp the ruling class, but genetically the invading tribes were a small population that merged with a large population.
The modern population of Italy is descended from the ancient Romans, which were descended from the more ancient Celts, just like the people of France (and to a lesser extent, Germany).
And I would associate that ancient population which colonized Western Europe with the Y-chromosome Haplogroup R1b [1].
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Now obviously this isn't the entire history. Genocides happened and displacement happened. (I even found an interesting study which found that descendents of ancient Etruscans are still around, but not on ancient Etruscan sites -- they've been displaced to nearby valleys [2]).
But on the average, it looks like the people who lived in France and Italy circa 500 BCE had a much larger genetic impact on the modern populations of France and Italy than the small invading tribes which displaced or merged with the ruling classes of those regions during the fall of the Roman Empire.
1. It is inconsistent with historical records. There's no evidence showing that the ancient Gauls, Celts and so on are of the same nationality as that of Romans. Their Y DNA are unlikely the same.
2. It assumes the invading Franks, Goths, and so on have different gene from the general public that they ruled over, i.e. Goths and Franks were not R1b, but there's no evidence to that. There's no evidence that the European medieval noble class were not R1b.
3. It is inconsistent with the common phenomenon of an invading ruling class's Y DNA quickly displace the ruled ones. A recent example is south American. R1b becomes the majority in a little over 500 years, displacing the original Q DNA.
4. The timeline does not work. R1b is a new gene originated about 10000 years ago in Asia. There's no way they are the one first colonized Western Europe. People with I, G and other far older genes have been there for a long time when R1b shows up.
The invading Romans did not displace the pre-existing population of Gauls in what is now modern France. The invading Franks did not displace the pre-existing population of Roman Gaul. The people who lived in France in 1600 were mostly descended from the people who lived there in 1600 BCE.
The people of modern Italy and France are descended from an ancient colonization of Western Europe. The invading populations of Goths and Lombards were able to usurp the ruling class, but genetically the invading tribes were a small population that merged with a large population.
The modern population of Italy is descended from the ancient Romans, which were descended from the more ancient Celts, just like the people of France (and to a lesser extent, Germany).
And I would associate that ancient population which colonized Western Europe with the Y-chromosome Haplogroup R1b [1].
--
Now obviously this isn't the entire history. Genocides happened and displacement happened. (I even found an interesting study which found that descendents of ancient Etruscans are still around, but not on ancient Etruscan sites -- they've been displaced to nearby valleys [2]).
But on the average, it looks like the people who lived in France and Italy circa 500 BCE had a much larger genetic impact on the modern populations of France and Italy than the small invading tribes which displaced or merged with the ruling classes of those regions during the fall of the Roman Empire.
https://en.wikipedia.org/wiki/Haplogroup_R1b#R1b1a1a2_.28R-M...
http://journals.plos.org/plosone/article?id=10.1371/journal....
http://brilliantmaps.com/the-genetic-map-of-europe/