I'm not sure if this is the correct section to post this, but if anyone can assist me with this concept then that would be awesome.

I love the genetic system in the Sims 2, I do, however, want to make some changes but I don't know how to do so. Everywhere I've read about the skintones say that it is a gene but it doesn't pass on like hair and eyes. If a sim with S1 skin and a sim with S4 skin have a child, they can be anywhere between S1 and S4.
Let's say two sims have a S1 and S4 parent and they both end up with S3 skin, if they reproduce then all of the kids will have S3 as well. Is there a way to make it more genetic? So that these sims would have children with a 25% chance of S1, 25% chance of S4 and a 50% of S1-S4?

In regards to hair genetics. Is there a way to get the hair traits to work like skin colour? For example if a black-haired sim and a blonde-haired sim have a child, that child will have the possibility of being either 33% blonde-haired, 33% black-haired or 33% brown-haired. Red hair could remain recessive.

Any help is appreciated. Thank you.

There's two hair/eye genetic mods here:
https://modthesims.info/d/641748/ev...r-included.html
https://modthesims.info/d/597849/mo...-24-jul-17.html
They make the eyes and hairs equally dominant/recessive, and you can make it so some are left recessive, so I guess that's about what you want? I think they're different, so pick one.

Eyes and hair are either dominant or recessive. Sims have two "genes". Directly from CAS, unless you've done some pacifier rolling and a bit of trckery, sims have two identical "genes" for the eye and hair they've gotten. A baby sim inherits one "gene" from each parent - if they're both recessive, one of those will show on the sim. If one or both is dominant, the single or one of the two dominant colors will show on the sim.

So a CAS couple who starts out with a sim with light blue eyes and blonde hair (both recessive) coupled with a black haired sim with brown eyes (dominant) will most likely have a black haired baby with brown eyes (but, say with recessive traits for blonde hair and light blue eyes). But if that sim is paired with a CAS sim who has green eyes and red hair, their babies have a good chance of getting both green eyes, red hair, light blue eyes and blond hair (plus brown eyes and black hair too), because the CAS sim only has recessive traits. Which means that through generations sims can have the chance to get other hair and eye colors than their parents, as long as you make sure to mix in some recessive genes. They can even get genes from grandparents and great-grandparents.

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As for skins, they don't work on a % scale, it's more of an availability scale. The baby can get any of the skins on the range between parent A and parent B (possibly mixing in some genetics from earlier generations, without saying it for sure). I don't think you can add mechanics that aren't already in place.

Geneticized skins are an option, but that only complicates the matter - those add skins between or before/after the range of the S1-S4 scale, so you'll end up with an even larger scale of skins.

The easiest if you want to make sure a sim gets a precise choice between two skintones is to use custom (and non-geneticized) skintones, because then there's just two choices to pick from.