You know that word game where you start with one word and change one letter at a time to make another? This is basically that, but with sims.
The goal of this challenge is to breed a sim with specific characteristics by finding partners that can pass those traits down.
Make your founder in CAS. You can freely choose their appearance and traits if you want. Then, create a second, randomized sim (the target sim). Hit the randomizer buttons until you get a sim that has a different hair color, eye color, and all different traits than your original (note: stick to natural hair colors for this one). Make note of what all of these things are, and then delete the second sim.
Your goal is to transition your bloodline from the characteristics of the first sim to the characteristics of the second. To do this, they must find partners that possess these characteristics and reproduce with them so that they can be passed down. Each offspring must choose a child and teen trait that your legacy sim also has. Their third trait must be one that they get from their other parent. Essentially, they are 'taking after their parents'. Your offspring will go on to find a sim that has another trait from the list and have a child with them too, and then pass down two traits as well while their partner passes on one. This means that each generation can get you one personality trait and/or possibly one or both appearance traits closer. You swap the traits out as you go until they have all changed.
So, for example , say that these are your original CAS sims:
Traits: Dance Machine, Creative, Outgoing
Appearance: Black hair, Green eyes
Traits: Kleptomaniac, Genius, Goofball
Appearance: Red hair, Blue eyes
If you are doing this correctly, it should take 3-5 generations to breed the correct sim (one new trait per generation + maybe more to get the appearance). Your founder sim should ideally try to find a partner that has the correct personality trait to pass on AND one of the appearance traits. So if the founder in the example finds a red-haired goofball, they should go for it. They could then have a Creative, Outgoing Goofball child with red hair and be two steps closer.
So you might end up with something like this:
Gen 1: Dance Machine, Creative, Outgoing, Black hair, Green eyes
Gen 2: Goofball, Creative, Outgoing, Red hair, Brown eyes
Gen 2: Goofball, Genius, Outgoing, Red hair, Brown eyes
Gen 3: Goofball, Genius, Freegan, Red hair, Blue eyes
Gen 4: Goofball, Genius, Kleptomaniac, Red hair, Blue eyes
1) You do not have to take a personality trait from the other parent if you already have all the right traits.
2) You can change your sim's gender settings once per generation (enabling your sim to get pregnant/get other's pregnant to match your potential partner's reproductive capability)
2) Offspring keep the hair/eye colour they were born with. No matching it to their other parent unless they inherited it naturally
3) You can have as many babies per generation as you like to get the right hair/eye color passed on. All children must follow the 'min. two traits from legacy parent, max. one from other parent' rule, but you can give them different combinations. Pick the offspring that carries on the physical traits best.
4) Add on other target appearance traits such as skin tone or body type if you wish
5) Age up your legacy sims as quickly or slowly as you like. You could draw this out into a full legacy playthrough or finish it in 10 minutes. However, you can't forcibly age up NPC sims outside of setting lifespans to short. So if you need Brown-Haired and Evil to finish, you are either going to have to wait till Max Villareal grows up on his own or settle for someone else.
6) Whether or not you marry/move in/date partners is up to you if you are playing a sim that can get pregnant. However, a sim must be born into your active household to count, so if your sim can get others pregnant they will need to move their partner in at least until the baby comes.
7) You can have a child with someone that has none of the desired traits and pass whatever they have instead down too. It will just make things take a bit longer.
8) If your legacy sim dies after the first generation you can swap in one of their siblings even if you have already moved the sibling out.
9) You fail if your original bloodline dies out or a child gets taken away. No neglecting toddlers that got the wrong eye color. You can age them up quicker if you want though
10) The challenge ends once your have bred your target sim and they become an adult with all the traits.