I don't buy that.
I'm a single race (as far as I know), but hope for a world where the races mix thoroughly and most of the world is brown.
I know of one person who is a mix of four "races" - Asian, black, American Indian, and white - and that's golfer Tiger Woods. It didn't seem to hurt him too much. He dominated his sport in his prime. From
What Is a Cablinasian? It's What Tiger Woods Claims as His Ethnicity :
Woods' Parents Both Multi-Ethnic
Woods' mother, Kultida, is a native of Thailand whose own ancestry includes Chinese and Dutch. She once referred to herself as "half-Thai, one-quarter Chinese and one-quarter white."
Woods' father, Earl, was an African-American whose heritage included white, Asian and Native American ancestors. Earl Woods once called himself, "half-black, one-quarter American Indian, and one-quarter Chinese."
So Tiger Woods' racial and ethnic heritage is predominantly black and Asian, but also includes white and Native American ancestry.