Architects were a rather weird civilization. You see, not only did they create servant races like demi-humans, but had also engineered themselves to be specialized in various areas. Here is the hierarchical order and their roles:
- White Fixer: They're effectively living archives and gods, tasked with ruling over the Architects. Their unique ability allows them to mind control those who are beneath them in the hierarchy. They have white skin with some gene-engineered marks for identification, white hair, light gray eyes, and usually a mesomorph build. They've been gone for quite a while by the time the story starts.
- Ginger Fixer: High-grade technicians and leaders, only having to answer to their higher-ups and the White Fixers. Hair color is ginger, eye color may vary and skin tone is white.
- Yellow Worker: Lesser technicians, skin color is yellowish-white.
- Brown Worker: Lesser physical workers for various physical tasks that are too varied for robots (gardening, for instance).
- Black Troubleshooter: The tallest and heaviest sub-species, designed for combat. They're just as mentally capable as workers but have several engineered traits that make them compliant with any order their higher-ups give them, though they can voice their concerns.
If this society seems racist, that's because it is. I never said Architects were the good guys.
So, at one point, fixers had infighting, which became a civil war, which ended up unleashing nightmarish creatures, called abnormalities, on the world. The remaining White Fixers fled to a city in the sky, hidden from everyone else. Later, they disappeared from there as well, only leaving an enslaved dragon behind, who was the core of their security system.
As for the other castes, ginger fixers, and demi-humans, they've forgotten their past and formed their own societies, making use of the remnant tech from the Architects.
Of course, there is still a White Fixer left (amnesiac, of course) who is the protagonist. However, it's been so long that attempting to use his mind-control powers on humans or demi-humans would fail, even if he knew he could to that. I.e: the Architects' genetic safeguards were eroded by nature.
But how? Architects used quantum supercomputers and nanotechnology to effectively erase the boundaries of organic life. The only taboo was the creation of a Singularity, which was only done at the end of the war when there was simply no other way of stopping either the pro-killbot or the anti-killbot camps.
Simply put, Architects' chosen a method of keeping their castes' genetic code pure relied on their society existing and actively maintaining that purity. The moment the fixers were gone, the safeguards also disappeared quietly into the night.
Architect society didn't even like inter-caste relationships. When another White Fixer in the story finds out that the protagonist is dating a demi-human, she gives him the same look you'd give to a zoophile or a furry, even though demi-humans are just as intelligent as Architect Workers by default. So, if this society didn't make the safeguards more "stable" it was because they couldn't without negatively impacting the castes' functions.
They could have made so that different castes were genetically incompatible, or have the baby's caste be either the mother's or the father's with no in-betweens. So, for what "practical" reason did they decide not to do either?