You’re thinking about genetics working the same way they work with humans/animals: ie, two donors, and then mixes from there, I think.
All genetic material is reprocessed and remixed by the mothergrub. There’s no direct lineage, between Ancestors and Descendants – the notion is cultural, but not factual.
All that happens is that, from that impossibly complex mixture of genetic material, every once in a blue moon, the mixes somehow produce an individual who shares 99.999999999999999999% of their genetic makeup with another troll, who also picked up the same sign they did. Do all trolls have Ancestors? Statistically? Yeah. Do all Ancestors are relevant? Not really. Agness’ ancestor was one of Dualscar’s unnamed slaves, for example, and neither Agness nor Eridan will never know because all records about Ethlyn’s existence have been destroyed. Plus only the longer lived castes even believe Ancestors are a thing anyway. Most trolls think it’s a myth.
Again with the numbers and the game of probability. Trolls, as a species, have a near infinite potential for genetic variation: overall size, horn shape, horn size, teeth/fang disposition, length of limbs, etc. Trolls come in all shapes and sizes and that’s determined by the slurry mixture the mother grub creates. The genetic material they provide is not structured, but it’s rather free-floating chromosomes suspended in stasis that react to another troll’s material during sex – if the required enzymes and hormones are presented, that is if the sex is legit black or red – and recombine into new bits and pieces that are still free-floating and in stasis when they’re delivered to drones/medbays.
THAT material is processed and recombined, again, by the mother grub, who uses the random chromosomes to build actual, functional chains of DNA from basic internal templates for each blood caste, filling in the blanks of acceptable variation – ie, what horn shape, what eye shape, what nose shape, what kind of teeth, etc – arbitrarily.
Who you have sex with has absolutely no impact on your descendants. That bit is propaganda by the Empire. So long as your relationships are biologically true – kismesissitude and matespritship – the material will be processed and your genes will go to be spread out to future generations.
Your descendants are nothing more than a quirk of statistical probability, where given the size of the Empire and the amount of eggs laid and wigglers raised, one day, maybe, someone will crawl out of the caves looking just like you, wearing your sign.
Inheritance is not a genetic thing, it’s a psychic thing all together.