the Plantagenet estimate is based on documents, the Genghis Khan estimate is just one of many possible guesses. My view is, if there really is a common DNA trace, it could literally be anyone, and is actually more likely to be some anonymous barbarian vagabond and/or undocumented immigrant from the steppe — or even some Central Asian ancestor who never left Central Asia but who had descendants who did.
Niall of the Nine Hostages likewise perhaps shows up in common DNA traces among (mostly) Irish-descent people, yet he’s otherwise known only from legend. Like Genghis Khan, there are no physical remains from which DNA could be extracted.
I’d guess that Mark Antony, Julius Caesar, and other famous Roman generals sowed wild oats all over Europe and the Med basin. JC’s troops used to sing a “lock up your wives and daughters” type song they’d sing on the way into a town. But think of all the literally millions of Roman soldiers who probably did pretty well wherever they went. And the slave women who were constantly available around the finer homes throughout the empire... trying to nail down (sorry) the exact source could prove to be problematical.
Thanks.
L