[SPOILERS] (sorry if I did the spoiler tag wrong, I'm new here) While the average individual won’t be able to distinguish between pure Viltrumites and hybrid Viltrumites due to dominant genes, powers, etc., it doesn’t mean hybrids can produce pure Viltrumites. Even a hybrid that’s 99.9% Viltrumite is still a hybrid only two purebloods can create a pure Viltrumite. Off-topic, but their pureblood species seems doomed. By the start of the series, there are only 40–49 pure Viltrumites left, and by the end, that number drops to 20–30. So how exactly do they plan to save their species? Thragg (and later Nolan) aimed to repopulate by breeding with humans or other compatible aliens to "save the empire," but this makes little sense. Thragg, an extremist for Viltrumite purity, advocated for this method despite it being impossible to produce 100% pure Viltrumites through hybridization. The only viable method would have been for the remaining purebloods to reproduce among themselves early on, hoping their genetics could avoid incest-related issues.