The game you're describing sounds like Millennium 2.2 (released as Deuteros in North America), a 1989 title where you colonize the solar system and genetically modify colonists for different planetary environments.
Clickbait rescue: “Millennium 2.2 (1989) — and possibly its 1991 sequel, Deuteros: The Next Millennium.”. Not released for C64, but (among others) for the Commodore Amiga.
You even had to genetically engineer your colonists, so they could withstand the environments. Fun game, but I can't find it via Google.
If anyone knows this game, please share. I'd love to play it again.