Mixed Races in futuristic dystopia

This might be better off in the Experts forum, but I thought I'd start here...

I'm writing a dystopia set in future North America, and I'm trying to use a slightly different form of racism (on the part of the characters) to reinforce the class divide that is at the core of the novel. I'm thinking that the lower classes (the heroes of the novel) are all mixed race... this is based on the assumption that racism will have declined in the future (so, it's not a total dystopia) to the point where there will be no barriers to inter-marriage or having children with people of all races.

But I'd like my upper class to consider themselves 'pure' and pride themselves on being able to trace their genealogy to precise geographic origins. I'd like these origins to be all over the world - like, I'll have characters of pure European descent, pure African descent, etc. But I'd like to subdivide it a little more fully, with the idea that the richer people will have worked so hard to stay 'pure' that they'll still be clearly Scandinavian, or Slavic, or whatever. There are some sympathetic upper class characters, but in general, the system is oppressive and cruel, and definitely in need of a good uprising.

So first question - does anyone see anything offensive in this general premise?

And second question - can anyone help me out with some visually distinctive non-European cultural/genetic groupings that I could refer to? I'm aware of the San people of Africa, who are often referred to as being physiologically distinct from their neighbours, but I'm not sure if they're distinct from ALL other African groupings or just those surrounding them. Are there visual distinctions between different groups of North American First Nations people? Are there distinctions among Australian Aboriginals?

And, finally, can anyone think of a genetically homogeneous group that would appear similar to people who are thoroughly mixed race? I'm thinking of having a character who is upper class and 'pure' but is repeatedly mistaken as lower class because whatever version of 'purity' he's displaying is physiologically similar to the mixed-race underclass.

Thanks for any help, even if that help is just telling me that the idea is fundamentally flawed for some reason!

It wouldn't be racism exactly, I guess. But it would be discrimination based on race, so...?

But, yeah, the major divide in this society is socio-economic. The race-based discrimination is secondary to that. I'm kind of looking for a way for the rich folk to justify their privilege and reinforce, to themselves, that they didn't just get lucky by being born rich, they're actually innately superior to the people they oppress. But I didn't want to go with standard, our-world racism, so I thought I'd try to put a twist on it.

Do you know which First Nations groups were known for different characteristics? And do you know whether those differences survive today, or have they been intermarried/extinguished?