I was reading about the Normans in Sicily yesterday and that led me to wondering how many people from Sicily getting 23andme et al tests are distant descendants of those Scandinavian-through-France settlers that would seemingly have been intermingling with the locals for hundreds of years of general Norman dominance of the area. But apparently nobody in Sicily seems to have whatever the genetic markers for “Northern Europeans 1000 years ago” would be. That led me to some assumptions/questions:
1: Maybe DNA of a Scandinavian-French guy in the Mediterranean from 1000 years ago may be so watered down it just doesn’t show up anymore in a person’s results?
2: Maybe Sicily was a lot less of a “Norman settlement” than places like Normandy and Northern England at various times or the various islands that were all a lot closer to Scandinavia. Convincing the family to move from like Jutland to Northern France is probably a lot easier than having to go all the way around Europe deep into the Mediterranean. Is there any academic estimation or speculation on exactly how many Norman settlers and soldiers etc were actually in Sicily around the ~11th century?
3: How much of the Norman army was ever actually Norman? The story of Robert Guiscard says he left Normandy with 5 riders then built a “roving robber-band” in Italy until he married an Italian woman for her Italian army. He eventually got strong enough to fight and defeat one Pope and then be made Duke by another Pope. But it seems like pretty much all of his manpower was Italian. After he was made Duke was there a push to bring in Norman settlers and troops and administrators or was he happy with his generally Italian retinue and army?
4: If there were a significant amount of Norman settlements and settlers in Sicily during Normal rule, did things go bad for them after the Germans took over? Did they fail to assimilate with the locals and eventually get wiped out/run out?
We don't actually know the precise size of the influx of Normans in Sicily. What's certain is that rather than relying on the size of the migration in order to make a lasting social and cultural impact, Norman influence in Sicily and the South was more a function of the influential roles that the Normans were able to carve out for themselves in social fabric and in political institutions. Indeed, the Normans were the most successful of all a manner of migrants to the South which could include Lombards and Occitans among peoples other parts of Europe. But these migrants nonetheless never replaced preexisting Greeks and Arabs who by far remained the largest ethnoreligious group until well after the Norman conquest (and even here, they didn't disappear - they just merged into a new, Southern Italian identity). In addition to piecemeal and admittedly confusing migrations of people to the South, the whole of the Mediterranean (and Southern Italy in particular) was all the while an enormous crossroads with all sorts of people coming and going, so I ultimately I think making heads or tails of any typical "Genetic Makeup" is a futile exercise.
Of course, this doesn't change the fact that a clique of Normans were enormously successful in first carving our an influential role for themselves, and ultimately seizing power the Principality of Salerno from which they embarked on a successful conquest of the whole of Southern Italy and of Sicily. But it's important to keep in mind that when the conquest of the South kicked off the Normans were already well-integrated in the local social and political hierarchy for over a generation. I actually wrote a little about that in this older answer which might interest you.
But what I'd like to insist on is that while a clique of Normans did successfully accumulate power in Southern Italy (and this ascending parable attracted other Norman adventurers who also made their way to the South, ultimately meaning we cannot discount the presence of a large ethnically Norman contingent in the Southern Peninsula) the near-continuous state of conflict which had created a space for the Norman's ascendancy in the first place had, tellingly, also attracted Occitan and Lombard adventurers. And let's not forget the conflict's primary stakeholders all the while continued to be local Greeks and Arabs (and Southern Lombards too). So what I guess my point is that given this landscape, the Norman Conquest ultimately had more to do with seizing political power at the top rather than creating any sort of widespread ethno-social change. In other words, it is more correct to say that the Normans would eventually become Sicilian, not the other way 'round.
I also wrote a section on Norman Sicily in this longer narrative on the wider history of Southern Italy as well as this shorter and much more to-the-point answer on the Norman Conquest of the South which also might interest you.