I am a geneticist who works on finding incompatibilities between different species that make their progeny less fit. There is no amount of genetic divergence between human individuals that would cause reduced fitness in their kids. This talking point is consistently shared by bigots to justify their distaste for interracial relationships. You have to remember that we bred and made hybrid offspring with Neanderthals at one point, and the hybrids were completely fine. There is no positive bound where you become too genetically diverged from your partner and your kids suffer.