> This is a silly argument.
My main argument was that the OP's supposedly clear-cut genetic category was not defined. You also seem to be saying that it's not clear-cut and defined. So why are you saying that my argument is silly, as opposed to the OP's?
> “ethnicity”
Neither the OP nor I mentioned this word, of course. You're shifting the goalposts. One could pick out a certain set of genetic markers, and one could do a clear-cut classification based on those. I'm firmly of the opinion that one should not, but in any case, that would be about genetics (as the OP and I discussed), and not about this word that you are trying to drag into this to deliberately muddy the waters.
> You can make the exact same argument about “species”
Proof by analogy is fraud, as they say. Even if the analogy holds. This one doesn't: There are fairly good ways of defining species objectively. For example, as the second sentence of https://en.wikipedia.org/wiki/Species says: "It can be defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction." I'm sure that there are a few exceptions around the edges, but this is much more clear-cut than any human divisions that you're trying to drag into this.