There have been a number of genetic studies[1] that can identify caste heredity going back to almost the time of the Vedas showing VERY little genetic mixing between the castes.
Oops, I may have chopped off a few characters. Yeah there were essentially only a handful of upper caste men gathering children with lower caste women and virtually no other mixing for thousands of years.
Wow, that is less intermingling than I would have guessed. I knew that caste had been there for ages, but it is still a bit astonishing to read that.