both have SOME scientific basis. Race isn't clearly defined but there are genetic traits that are common to people with shared ancestry. For lack of a better term, that's race. If I need a bone marrow transplant, I won't be able to get it from a Korean.

While both race and gender are fluid, they're fluid in different ways. We all have biological parents of different genders, giving us the capacity for both ends of the spectrum and everything in-between, but a child born to two white people is going to be white. Race is only fluid when there is mixing.