Lactose intolerance being a personality trait is just Disrespectful.

Honestly medical conditions would work great as "reward" traits. I'm not saying having a chronic condition is a reward, god no,  I have some myself and would like them gone, but purly within the game's mechanics that would allow you to add minor traits to your sims that don't take away a personality trait slot. I am lactose intolerant but I never use this trait in game because it isn't a personality, yeah, and I'm not giving up one of three trait slots. Now, reward traits can be easily added in game and give small gameplay effects. Most of them are good, but I think the mechanic would absolutely work to add minor traits to sims that make the game harder, too. Let us set sim personality in cas and then hand us a few "reward" traits to further define the characters.

Or, that's probably better than the reward trait idea, make them like quirks for infants/toddlers, but on adults. Minor traits that can be added by the player and/or aquired through gameplay. Quirks could be illnesses, preferences, behaviours not tied to traits etc.