My Sim recently gave birth to twins. One of them is a redhead. Both parents have the recessive gene for blonde hair. Could this be what’s causing it?

Are there any redheads further back in the genetics? In TS2, babies can inherit genes from grandparents (via parents), possibly even further back. One sim could inherit a recessive gene from a grandmother, but with the right combination of recessive and dominant genes it wouldn't be expressed until their children have children.

(edited to specify)

You can find out what genes everyone actually has by typing ctrl+shift+C and then "simDNA <first name>" to get the simDNA for the sim with that name who's on the lot. If a sim with blond hair also has a gene for red hair, they might pass the red-head gene onto their child and not the blond gene.

Only figuratively.
Sim babies can't magically inherit genes from their grandparents or great-grandparents. Hair, skin and eye colour each have two genes in Sims 2. A newborn Sim always gets one of those gens from their momma and one from their poppa (unless there's an alien abduction, then one gene from the daddy and one gene from the alien).
The game then decides which of these two genes gets expressed. If black or brown are present for the hair they are always dominant over red or blond. So it can happen that a blond Sim passes down a recessive blond gene to their black haired son, that son can pass down the recessive blonde gene to their brown haired daughter, who then might marry a red haired guy and have a blond baby with him. The baby has inherited its mother's recessive blonde gene, which she has inherited from her father, who has inherited it form his father.
Babies only ever get genes from their parents, but certain genes can be passed down many generations within a family line until one day they get expressed again. In my current neighbourhood both Cassandra Goth and her brother Magnus have inherited Mortimer's recessive blond gene, who has it from his father, Gunther, who has it from his father Victor (who was blond). So figuratively, a blond child of Cassandra's could be said to have inherited "Victor's" blond gene, but in actuality, it has inherited Cassandra's hidden blond gene.

So the only way, normally, the OPs baby can have red hair is if one of the parents has an unexpressed red gene.

Indeed. Red-hair gene could have carried over trough generations but wasnt expressed, and baby inherited it. One of the parents must have it, despite not expressing it., for them to be able to pass them onto their child.

Damn I love TS2 genetics. Wish it was available in newer games. I would play TS2 for genetics alone.

OOPS ! Maybe the mother of the baby with red hair was playing around with another Sims not her husband and got pregnant. Just saying Lmao

like another poster told, sims do not inherit genes directly from grandparents or further; just from parents.
hair genetics; black and brown are the dominant colors, blond and red are the recessive colors.
2 possibilities for red hair to be expressed::
-both of the newborn's alleles being red
-one allele being red, other allele being blond.

(I guess I wasn't clear enough in my first post).

Genetics are based on a simplified genetic system in TS2, so you can create some interesting families if you just plan a little ahead.

In a non-modded game, the genetics will be passed on as follows:

Hairs:
Black and brown are always dominant, if both are present the game will choose between them.
Blonde and red are always recessive, if both are present the game will choose between them.
Custom hairs in the custom bin... no idea, though I think they're based on the base hair color assigned to them.
Parents can pass on the unused recessive or dominant gene to their children.

Eye default colors:
Brown and dark blue are always dominant, if both are present the game will choose between them.
Grey, light blue and green are always recessive, if two are present the game will choose between them.
Custom non-geneticized eyes are always dominant over defaults. If two are present, the game will choose between them.
Alien eyes are usually dominant, but they can sometimes mix in with dark blue and brown.
Parents can pass on the unused recessive or dominant gene to their children.

It takes at least two generations to remove a hair or eye color entirely from the gene pool.

If recessive genes are passed on along with dominant ones, they may tend to skip two or more generations before they show, because the dominant always shows on the sim.

If you have a blonde, green-eyed sim, and a blonde, grey-eyed sim, there aren't any dominant genes in their gene pool, but they can still have a red-haired child with light blue eyes if they have the genes for it. If you after a few generations have a black-haired, brown-eyed sim, and a brown-haired sim with dark blue eyes, they can potentially get a red-haired, grey-eyed child.

Examples below on how recessive genes for hair and eyes can skip several generations ingame, and how dominant genes can be filtered out. (With "CAS-sims" in the pictures I mean auto-created or taken from the sim pool, without use of the pacifier-roll. As pointed out below, pacifier-rolled sims in CAS can get mixed genes).

Nevermind the pictures weren't working, they do now.

Aren't custom hairs always dominant? Or is that just skintones and eyes?

Custom hairs dominate over everything - however, unlike binned hairs, the exact hairstyle will be used for a child of the same sex as the parent with the custom hair. So, if the mother has the custom colour, all her daughters will have that exact style, while her sons will have something else (I think it might be the base colour but not 100% there). The same applies to any sons of a father with a custom hair colour.

Regarding CAS sims, the pictures that were posted by simmer22 are only correct (ie CAS always have 2 identical genes) IF the player created from scratch and didn't use the Create-a-Child function. If they made two (homozygous) sims, created a child from those sims and then grew the child up to become the new adult, that sim can be heterozygous and carry different genes that aren't expressed. I've given CAS sims recessive genes that way before, and had surprises generations down the line.

TS2 genetics are both very simple and very complicated all in one. That's why I love them!

Which is why absolutely ALL my hairs are geneticised - or all sims start out with a geneticised hair anyway. I have hats in the custom bin.

I was disappointed with the TS3 version of genetics. Much prefer TS2 in that department.

They used to be dominant in the first EPs, but after Seasons (I think? The one that stopped the "every kid get the same hair" bug) I'm not so sure. I don't put a lot of custom hairs on my sims in CAS, so I don't have a clue.

I think hair colors may now be decided by eyebrow color, so if a sim has a custom hair but brown eyebrows, they'd really have brown hair. Either that or the base color in the recolor file for the custom hair (they always have red/brown/blonde/black/grey in one of the resources). I've noticed their eyebrows sometimes change when putting on a custom hair, so that's a possibility.



CAC/pacifier sims would probably be akin to generation 2 in my pictures (grandparents)
CAS sims not made with the CAC/pacifier always have fully dominant/recessive colors.



Ugh, it's one of the reasons I grow tired of both TS3 and TS4 so fast, and it's my main complaint with both games. Babies in TS2 actually have proper genetics, and inherit both eye color, hair color and skin from their parents, visible at birth. Since they have the same face texture as their parents' skintone, they look a whole lot more like their parents, too. TS3+TS4 babies don't, and they even have entirely different skintones and body structures from their parents, to the point where they don't even look like they're the same species.

I might have to do that with hats myself, then my sims can still have them rather than just having them hidden...
I also like all my regular colours to be fully binned and family-linked, and the only time I used custom colours directly out of CAS was to make some "alien sims". All human sims get a geneticized hair colour.

Indeed, that's a good way to refer to it. I just wanted to point out that you can make the generation 2 sims in CAS - some people might not be aware of it

I've been playing TS4 as well as the TS2 lately. While TS4 genetics seem to actually be a little better than TS3, TS2 blows them out of the water, as usual. The older it gets, the more I realize what a masterpiece of programming it is.

TS2's genetics system is amazing. It's fun to make two Sims with exaggerated features and mess around with Create-a-Child. Also, previous experiments with inbreeding.

This was a mistake. THIS WAS A MISTAKE.

You will find this can all be breed out by the fourth generation too. yes, sim babies can get genes such as red hair from a grandparent. My most diverse family has 3 kids, one has brown, another red and the third blond hair. I love TS2 genetic system!

I think it's time one of them has to do explaining while the other interrogates that sort of mystery.

It happens in RL too
My grandson, born with dark brown hair, just turned into a redhead.
Mom and dad have dark hair.
Grandparents have dark hair, except for one, who is blond.
Son-in-law, though, has not one, but two red-haired brothers.
Grandson also has blue eyes - neither parent does, nor does any grandparent - but great grandmother does and ALL her grandchildren (except for my son-in-law) have blue eyes!
So Sims sound very realistic to me

To put it simple, Sims just uses the mendelian inheritance system which most of us probably had to deal with in biology at one point or another. I still remember having to draw those diagrams.
I think this is also why rolling the pacifer actually even makes sense (not only in the game programming logic). Each time you roll it, you combine all the genes new (or make way for a new combination to follow it).