I don't see how there's any way that some Jews expelled from southern Italy could contribute enough to Germans to make them 16% Ashkenazi, plus all the other groups which show it. By that time the laws against intermarriage between the two religions were Draconian, and that wasn't just on the Christian side. A Jew who married outside his or her religion was considered dead. Families were still sitting shiva for such people when I was in university. It was also just extremely dangerous, even for a Christian woman who converted and married a Christian, because they were never really accepted, as all the auto da fes of Conversos in Spain shows. For a man, in addition to becoming part of a persecuted, despised minority, he would have had to endure circumcision as an adult. It may have happened in individual cases, but I just don't think it would have happened to that degree. In addition, to the best of my recollection, most of the Jews expelled from Italy went to Ottoman lands and/or North Africa.
IBD sharing tests tell the same tale. No Italian group has any IBD sharing with Jews, certainly not within the last 1000 or so years. The IBD sharing with Germans/French is similarly just not there. The only at all significant sharing is with Poles/Russians, which makes sense given the history of those countries. Anyway, while this isn't about the Ashkenazim, I think these percentages point out a big flaw in the Nat Geo algorithm. Of course, every commercial company, imo, has flaws. This type of thing just isn't easy to do.
The older descriptions for the Tuscan reference population explains that the Lombards contributed to increasing Northern European ancestry in the 6th-8th centuries. This ancestry would be shared with people in Northern Europe.
Lydia Ramsey is Scandinavian, so if you look at how they break down her ancestry. It makes sense as to why Northern Italians have an increase of those components. Thus that would account for the increase to 9% Northwestern, and 12% Eastern Europe components, via Lombards, in Tuscans.
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Yes, I agree that the Lombards had a disproportional effect on Italy north of Rome (i.e. the Italian cline), but it's not only the Lombards. (Also, if the Lombard yDna is indeed majority I1 and R1b U-106, as some have speculated, the amount of actual "Lombard" autsosomal dna may not be very large.) It's also the Celts to some degree and the Liguri and Italics and Veneti as well.
No they don't, they only consider Northern France to be Northwestern European, a few other countries. The map is misleading; here's the description.
Well, that's good to know. It would be really bad if they were ignoring the Gascony sample, which is extremely low in "steppe" ancestry, and much more like the Basques or northern Spaniards than like the people of Brittany or Normandy, for example. They should really adjust the map so as not to mislead people.
I'm pretty sure these are based on historic populations, not just sampling from Southern Italians. It includes migrates from the north pushing southward. At least that's what the description indicates. But I think it's worth investigating to know for sure.
All any of these commercial companies are using is modern dna. In the case of Italy there really isn't even any ancient dna to use, other than Oetzi and Remedello, which are from roughly the same period and from far northern Italy. We have no Lombard dna from after the fall of Rome, or Gallic dna from the first millennium BC, or Greek/Italian dna from Naples or Calabria or Sicily from the first millennium BC either. What these companies are doing is what the creators of the gedmatch programs attempt to do, which is to look for genetic clusters. However, they're "modern" genetic clusters, the product of all the migrations that went before, and can be used to intuit specific historic migrations very imperfectly.
I guess the only way to really know is to e-mail them. Care to help me outline a few bases to touch upon?
Sure, no problem, although it will be difficult for me to be more specific than I have been given that I never took their test.
I do understand your frustration about all of this, but to some extent, in terms of genetics, ethnicity is a construct that is time dependent. Was Oetzi, were the people of Remedello "Italian" yet? When was the moment in history we're going to use? Will we use, when we get the ancient dna, the end of the Bronze Age, or perhaps the first millennium BC.? That's when Ralph and Coop found the IBD sharing between "Italians" and other groups to largely end, except for a pulse from the Balkans. Were the Celt-Ligures of the first millennium BC "Italian"? How about the probably "mixed" inhabitants of the city-states of Magna Graecia? When Julius crossed the Rubicon, he was crossing into "Italia" according to their maps, yet how different were the Romans of Lazio during the Republic from the people in the Po plain? How different were they from the southerners of that time? We just don't know yet.
As to something you wrote in another post, National Geographic may not do what 23andme does, and use their own customer base, and therefore Italian-Americans as their reference population. However, they obviously are using southern Italian samples from somewhere. That would be a good question to ask, btw. Which southern Italian samples are they using, from where and collected by whom?
I think it's pretty clear that's where their centrum for "Italian" is located. It's less clear to me that this is a better choice than, say, Tuscany, or Umbria, but as I said these are subjective decisions to some degree.
Apropos of all this and as a matter of serendipity, Razib Khan has posted about why the results from different commercial companies vary so much. I'm going to start a thread for it. Here it is:
https://gnxp.nofe.me/2017/08/29/why-do-percentage-estimates-of-ancestry-vary-so-much/