Originally Posted by Vixen
If you have a European noble somewhere in your ancestry in the past 300 years (~10 generations) you're almost certainly descended from John I Lackland, because he had five quite fertile kids who lived in five distinct areas of Europe and their children were prize catches for noble houses throughout the continent. John Lackland was a great-great-grandson of William the Conqueror.
Statistically you'd expect this to be true for any European or American by now. If not it will be within a generation or so.
Here's another thought experiment: suppose every descendant of William the Conqueror had 2 children who lived to procreate, and had them at an average age of 30 years. Rounding down that makes 31 generations, a theoretical total of over 2 billion people. There would be some imbreeding, but the average was certainly above 2 children too. William the Conqueror had seven children, one of his daughters gave birth to 11 children, five of whom ruled various fiefs in Europe. His other children procreated too, this was just the most impressive one of them.
Find me someone of European ancestry who can prove he's not
related to William the Conqueror and we can talk.