Its pseudoscience.
If races were a "real" biological thing, you'd expect them to work kind of like animal breeds.....clear categories where members of one group share a specific set of traits that clearly separate them from another group. Like how a Chihuahua and a Great Dane are pretty obviously different "types" of dog. Remember your German Shepard AI post?
When human DNA is examined however they see no clear lines. If you traveled slowly from, say, Nigeria to Egypt to Turkey to Germany, the DNA of people along that path changes little by little the whole way. There's no spot where you can say "okay, HERE is where one race stops and the next one starts." It's all one long gradual blend, not separate chunks.