What is the genetic basis for the one-drop rule? What is the genetic basis for Barack Obama being black and not white? What is the genetic basis for someone with three Jewish grandparents being Jewish but not someone with two Jewish grandparents? What is the genetic basis for an Irish person being white in 1900 but not in 1800?

No one denies that there are genetic differences between races, but the categorizations are completely arbitrary. Someone who is one-quarter black is more genetically similar to a typical white person than a typical black person yet they are still classified as black. That is what is meant by race being a social construct.