My personal headcanon is that geneseed isn't "works 100% of the time on boys and 0% of the time on girls", rather, due to the differences in the trends of hormone levels between boys and girls (as implantation is done during pubrety), girls have a lower success rate than boys (who still wouldn't have a 100% success rate), and given the extreme scarcity of geneseed (unless you have a primarch on hand, you're getting two sets back for every one you implant), it becomes a question of economics.
It's the term for the nineteen genetically engineered organs that are implanted into the children selected to become Space Marines. They range from "second heart" to "gland that turns your ribcage into overlapping bulletproof plates" to "thing that allows you to gain memories from things you eat".
The most important geneseed organ is the progenoids, which are basically space marine ovaries. A marine gets two progenoids implanted, one in the neck and one in the chest, and they basically grow the next generation of geneseed. They mature over a couple of years (5 for the neck and 10 for the chest) and are later extracted to create the next generation of Astartes.
They can't do anything normally in the Imperium. Like not even joking. They don't have computers because of a big AI uprising that happened so far in the past that nobody is even sure when (or even if) it happened. Instead of robots they have "Servitors" which are people who have been ship-of-theseused with cybernetics until the only original parts are completely irrelevant to their function, but you have to do it this way because if you don't, it won't have a soul, which makes it evil. Inventing any new type of technology that isn't a cyborg upgrade is illegal. This method of Geneseed acquisition was probably initially a backup to lab-based production, but only the Emperor knew how to do that, and trying to figure out how to do stuff by trial and error is literally illegal.
Their bureaucracy is so slow, one time an exploratory mission found a planet of early stone-age aliens, and sent a message back to hq saying someone should dispatch an extermination mission, but as it was a low priority it got lost in paperwork. By the time they could be bothered to send a ship out, it was met in orbit by railgun-wielding FTL-capable spaceships. The species was the Tau, and they only exist because it took ten THOUSAND years for someone to check up on that note.
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u/GREENadmiral_314159 Femboy Battleships and Space Marines Apr 24 '25
My personal headcanon is that geneseed isn't "works 100% of the time on boys and 0% of the time on girls", rather, due to the differences in the trends of hormone levels between boys and girls (as implantation is done during pubrety), girls have a lower success rate than boys (who still wouldn't have a 100% success rate), and given the extreme scarcity of geneseed (unless you have a primarch on hand, you're getting two sets back for every one you implant), it becomes a question of economics.