Technically, I could do it either way, as I have my own original codebase for generative and can code to any specs a client has. Most clients want control over the process in terms of being able to decide which traits are the rare ones (i.e. golden fur on a cat or something when the rest of the cats have dark fur). In other words, most teams have their artist specifically draw rarer traits with that intention. And so we begin with a spreadsheet for the initial round, and then we look at tests and we dial things in further from there if needed. But sure, if you had another way (and there could be legit reasons for any imaginable rarity scheme), we could do it that way, too. You tell me the logic; I translate that into code! :-)