So, you want to learn how to alter genetics so that, say, your two brown-eyed CAS Sims can have a blue-eyed baby? First things first, you need to go into SimPE and open the file of the hood those Sims live in. Here, have a picture - this is where you need to go. Click on "Neighborhood Browser" and it will open the hood for you.
Now, in the window that says "Resource Tree", click on "Sim Description (SDSC)" to find the Sims whose genetics you want to alter. For this tutorial, I needfed to find out what the values for the default genetics were, so I created five test Sims with all the EAxis hair and eye colours and moved them into Strangetown. They're circled in red below.
(I took a few naming hints from the in-game tutorial hood. Can you tell?)
Now, I've opened the first of my test Sims' files in SimPE. But what's this? There's not one mention of Tutorial Jack's genetics in the SDSC! However, we can find the SDNA file from here. Click the button that says "More" in the SDSC, and choose the option that says "Open SimDNA" to go to your Sim's SDNA file.
Why not just click on SDNA in the Resource Tree, you ask? Simple- all SDNA files have the same name in the Resource List, making it pretty tricky to tell which file belongs to which Sim.
So here we are in Tutorial Jack's SimDNA file. There are two tables, one for dominant and one for recessive genes. The string of letters and numbers beside each one is the value of that eye, skintone or hair colour. Note that the dominant and recessive genes for each display the same value - Tutorial Jack is a CAS Sim, so he carries the same genes on both sides.
To change the value to a different skin, hair or eye colour, you need to replace the value in the "recessive" box (or the "dominant" box if you want to change the trait the Sim actually shows, for example if you want to give a premade Sim a different hair colour). Here are the values for all the Maxis traits, for easy cutting and pasting. You'll need them, especially for the eyes. :)
Brown = 32dee745-b6ce-419f-9e86-ae93802d2682
Dark blue = 2d6839c5-0b7c-48a1-9c55-4bd9cc873b0f
Light blue = e43f3360-3a08-4755-8b83-a0d37a6c424b
Green = 51c4a750-c9f4-4cfe-801c-898efc360cb7
Grey = 0758508c-7111-40f9-b33b-706464626ac9
Black = 00000001-0000-0000-0000-000000000000
Brown = 00000002-0000-0000-0000-000000000000
Blonde = 00000003-0000-0000-0000-000000000000
Red = 00000004-0000-0000-0000-000000000000
S1 Light = 00000001-0000-0000-0000-000000000000
S2 Tan = 00000002-0000-0000-0000-000000000000
S3 Mid = 00000003-0000-0000-0000-000000000000
S4 Dark = 00000004-0000-0000-0000-000000000000
If you look at the values in this picture, you'll notice that the recessive eye gene is now the value for grey eyes. Well, you probably will, I've circled it in red for crying out loud. :) Just paste in the string for the colour you want as the Sim's recessive trait, commit and save. Repeat for as many Sims as you want or can be bothered to do it for, and go try a few genetics experiments if you're so inclined.
This can also be done with custom skins and eyes, but you'll need to find the values yourself with SimPE. Open the package containing the skin or eye you want, and look for the "family" value in Skintone XML (for skintones) or in Texture Overlay XML (for eyes). That's what you need to paste into the SDNA boxes if you want a Sim to be able to pass that custom skin or eye on to the next generation.
That's it from me for now. I hope this was helpful to you Starr, and to anyone else who has ever wondered how to actually do this.
ETA: I've done a little more research, and I wanted to add some advice on the SkintoneRange field in SDNA. If you are altering skintones, I would recommend changing that field to the same value as (I think) this field is used for the genetic range of skintones. I don't know what leaving it as-is when changing skins would do to genetics of the Sim's offspring, but the advice I found was to change both Skintone and SkintoneRange fields when altering skintones.