Plant Tycoon apparently has no genotypes as we know them. The game follows the old Victorian-era idea of "blending inheritance" that was refuted by Mendel's experiments. The fact that "hybrids" are true-breeding in Plant Tycoon violates Mendel's First Law. It's a fantasy game with a mathematical puzzle behind the "crosses," not a true genetics simulation.

If Mendel's rules would apply, then in your example of the ball cactus x maple => maranta, the two alleles for ball cactus and maple would need to be co-dominant so that their combination gives you maranta. You should then get 25% ball cactus, 25% maple, and 50% maranta when you self-pollinate the hybrid maranta. Maranta should not be a true-breeding "species" in this case, but in Plant Tycoon it is.