GMO Cells can be made with a specific Gene in mind, rather than faffing about with weights and chances and such.

They are made in a low temperature Advanced Incubator. Success chance is lowered with Overclockers, but raised with Chorus Fruit.

The recipes can be seen in-game with EMI, but many modpacks don't have it. While I work on JEI support, you can see the list of default recipes here:

Cell Growth recipes

These recipes take a Potion of Cell Growth set to a particular entity type, plus another ingredient.

These recipes don't let you get any unique Genes, they just give you a way to get common Genes without relying on randomness.

Mutation recipes

These recipes take a Potion of Mutation set to a particular entity type, plus another ingredient.

These are the only way to get some of the more unique Genes.

Do note that after getting a successful GMO Cell, you can duplicate it with a Potion of Cell Growth!