This was flagged, but I think mistakenly by people who think you're joking or taking a dig. The point is there weren't 70,000,000,000 people alive in Italy back then (or, you know, anywhere ever), so it is very likely that Dante shows up in multiple branches of this person's ancestry.
I am not sure that is a correct definition of incest, as it more commonly refer to breeding between family members or close relatives, with the definition of close being up to the legal system in place. Just two people with a common ancestry would be a too board definition to be useful.
But it is correct that 1 / 70,000,000,000 is the wrong numbers of the genetic contribution. In order to find the correct number one would have to determine the coefficient of relationship of the average couple. It is going to a much smaller number, but at the same time far from actually incest.
That said, incest law in Italy is a bit weird. It is legal, except if it causes a scandal.
You would also have to know whether adultery took place which resulted in a child and whether any of the 'descendants' were adopted. It would be less work to disinter Dante and compare the DNA directly.
Also, in fairness to the poster, incest was in quotes.
I believe you're close to correct. According to a paper in 2013 by Graham Coop (an evolutionary biologist from UC Davis) every living person of European descent, if you trace their family trees back 1000 years, likely shares the same list of ancestors. And that list of ancestors is more or less all the people alive in Europe at the time who had descendants surviving to the present -- about 80% of the people alive at the time. Joseph Chang, a statistician from Yale, has also written a great deal on the subject.
Not necessarily. If you are descended from historically insular communities (like Ashkenazi Jews, for example) or geographically distant communities (take Native Americans for a clear example) it is unlikely you have Dante as a direct ancestor.
You are right that in the infinite time limit, assuming nonzero mixing, either everybody or nobody will be Dante’s descendant.
Ashkenazi Jews are an insular community but I don’t think they’re insular enough not to have Dante among their ancestors. They have Polish/Slavic mitochondrial DNA among many other lines, and the explosion of that population was still in progress in Dante’s time. It’s entirely possible there’s no trace of Dante’s actual DNA because an ancestry share that small can just disappear by chance. But if he has living descendants he has living Ashkenazi descendants.
