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None of these statements are meaningful without a definition on your part of what it means to be "Ancient Roman" in a genetic sense. Do you mean, say, a handful of villagers living in the area of today's Rome on 21 April 753 BCE? I'll grant you that those were possibly a homogeneous bunch. Anything later? Definitely not. Trying to pretend that there is something like a clear genetic definition of "Roman" is... what d…
This is a silly argument. Biology is inherently messy and “ethnicity” is just a fuzzy clustering. You can make the exact same argument about “species” - because that too is a human abstraction that breaks down in biological reality because there are no clear lines.
There's a clear line in this case: A species is a group of any living beings that can produce fertile offspring.
Donkeys and Horses are separate species because they breed and produce Mules which cannot reproduce.