[cb18]

There are no genetic markers that taken together say "this person is black" or "this person is white".

Actually, there very much are.

I think what you're trying to say is that not all black people have all the "black people genetic markers," and likewise for other groups. This is true, but this is not necessary to distinguish well-defined groups.

Family resemblance is a philosophical idea made popular by Ludwig Wittgenstein

It argues that things which could be thought to be connected by one essential common feature may in fact be connected by a series of overlapping similarities, where no one feature is common to all.

https://en.wikipedia.org/wiki/Family_resemblance