Futagen is simply a way to say Futanari Mutagen a tiny bit quicker. The mutagen is present on all futas in the stories, and though it’s not really spelled out, it always has some role to play in the worldbuilding.
- In Rocket girls it was a malicious virus that their enemies deployed to wipe them out over a couple of generations, but ended up with a futa population instead.
- In Angel Girls the mutagen is the gift of God, all angels have it and that’s why they’re commonly treated as male.
- In Bronze Girls, same thing, it’s due to them having a piece of ichor, of divinity. Demigod humans are always futa, and demigod monsters have some latent part of the gene that makes them futa as well.
- In the rest of the stories the futagen is more conceptual, like in Cyber girls where it’s just a cybernetic prosthetic. And in Steam girls, same thing but more steampunk. In Parasite Girls it’s pretty obvious, the futagen is the alien parasite.