A Look at Genetics in The Sims 3

A Look at Genetics in The Sims 3

Note: I have not found any facts about the genetic system from someone who specifically worked on it, so I will use what is showcased in the game alongside with some comparable examples. Also, there may be inaccurate information though I really did try my best in researching. Some conclusions are just my own interpretation of how I think something works compared to how it may actually work. I am not a biologist or geneticist. Regardless, this sort of subject matter is both fascinating and fun for me. I will share what I learned in case someone may find this just as intriguing.


Case Study 1: The Rumbaa Family

Nathaniel and Trinity are founders I created in CAS and they have 15 in-game born children. From the oldest to the youngest are: Jonathon, Carlston, Genesis, Paige, Ryne, Hunter + Choice (Fraternal Twins), Velvet + Jules (Fraternal Twins), Dallin, Faith + Ryin (Fraternal Twins), Alex, Madison and Olivia.

(Sidenote: The first and second pair of twins I used a mod to get but left the gender selection at random. The third pair of twins happened without my intervention, if I remember correctly.)


Figuring out the parent’s genotypes and the children’s phenotypes using the Punnett Square

By having children, it may be possible to figure out the parents’ or founders’ genotypes in order to see the possibilities of the phenotypes they may produce. Of course not all possibilities may be shown such as codominance or incomplete dominance when using the Punnett Square.


Nathaniel and Trinity Possible Hair Color Genotypes and Their Children’s Possible Phenotypes


More or Less.

50% chance for inheriting Brown hair and is heterozygous like the parents.
25% chance for inheriting Black hair and is heterozygous like the parents.
25% chance for inheriting Blonde hair and is homozygous recessive.

(Since more children have brown hair, it makes sense to count Brown-black over Black-brown.)


Heterozygous means a person having two different traits for the same gene such as hair color. For example: Having black hair and carrying a recessive trait for blonde hair. It is always a dominant and recessive trait combination such as Bb. The capital letter represents the dominant trait and the lowercase letter represents the recessive trait.

Homozygous Recessive means a person having two of the same recessive traits for a gene. For example: Having blonde-blonde hair is written as bb. Both lowercase letters because it is an inherited recessive trait, one from each parent.
(Sidenote: You will notice in the phenotypes instead I use a dominant version of blonde such as Blonde-blonde instead of blonde-blonde on purpose. That is because I believe what an individual inherits and shows up on them becomes their dominant. I will explain again briefly later on.)

Homozygous Dominant means a person having two of the same dominant traits for the same gene. For example: Having a genotype for only Brown-Brown hair is written as BB. Both letters are capitalized.

Dominant vs Recessive Traits

What you see, is what is called the dominant traits on a person. Hunter is the only child with blonde hair and for him blonde is his dominant hair color. His genotype is likely Blonde-blonde.

Recessive traits are what some people carry in their genes to pass on to their descendants and are traits you cannot see on them. For example: Nathaniel and Trinity being able to pass on a recessive trait of blonde hair to their son Hunter. Despite the parents being founders, it seems the genetic system creator(s) takes into consideration the ancestors instead of a literal generation 1 approach. I suspect this means that founders’ are randomly assigned recessive genes and is a hidden feature within the core of the game that cannot be manipulated because that part is not open source.

This is why offspring may have a different eye or hair color (excluding mutations) than their parents whether founders or not. And of course, just because some Sims’ are carriers of recessive genes doesn’t mean they will always show up and sometimes if it does then not until the next generation or later. Also, any hair or eye color can be considered dominant or recessive.

Exhibit A

According to the director and writer of The Incredibles, Violet’s black hair is a recessive gene.

“Mr. Incredible has blonde hair, Mrs. Incredible has brown hair, and their daughter Violet has jet black hair. What's the deal with that? Are we supposed to think she dyed it?

No, it's a recessive gene. We will be creating the genealogy on the website someday. Stay tuned. It's a very complicated genealogy.”
(http://movies.radiofree.com/interviews/theincre_brad_bird.shtml)

https://i.postimg.cc/T2SftqPm/image-1c6a9b5d.webp

Bob could be Blonde-blonde or Blonde-black. Helen could be Brown-black. Either way, there is a possibility for a child to inherit black hair whoever inherited the gene from their ancestor if not both. I suspect both might have black as a recessive gene. I have not found any official genealogy of the Parr family so I do not know. I just know that it is true to life and possible for Violet to have black hair depending on her parents’ genotype.

https://i.postimg.cc/ZRc7XsYx/Bob-and-Helen-Possible-Genotypes.png


To determine the accuracy in percentages, it is best to go by in-game data. Of all the Rumbaa children:

9 children have brown hair.
5 children have black hair.
1 child has blonde hair.

Which translate to:

60% chance of a child coming out with brown hair.
33.3% chance of a child coming out with black hair.
6.7% chance of a child coming out with blonde hair.

Each time a child is born, those percentages may change and needs to be recalculated.

To find the statistics for your Sim’s offspring then count all the children and that will be the denominator. Count how many children have x hair color and y hair color and z hair color if applicable. Turn each statistic into a fraction and then reduce if necessary. There are 9 children who have brown hair out of 15 children altogether. So I get 9/15 which reduces to 3/5 then use long division which I got 0.6 then 0.6 X 100 = 60. You can use any calculator for the quick answer as well.

Nathaniel and Trinity Possible Eye Color Genotypes and Their Children’s Possible Phenotypes


11 children with green eyes
4 children with blue eyes

73.3% chance for green eyes
26.7% chance for blue eyes

Skin Color



Skin color, like hair color, is a polygenic trait meaning there are multiple genes that determine the color. There is such a broad range in skin colors all around the world, so it is nice to see TS3 does this rather well and is a huge improvement and advancement compared to TS2 limited presets. All 15 children vary from light to dark and showcases TS3 ability to handle offspring of parents who have different skin colors.

Sex Determination

9 sons
6 daughters

60% chance for a son
40% chance for a daughter

Only women have an X-chromosome and men have either X or Y chromosome to determine the sex of the baby at conception. I think this is also the case in TS3 and the sex is locked in place at conception.

XX = Female and XY = Male

The 3rd Generation: Grandchildren

Note: Some things may change as the generations grow and I am sure new discoveries to be observed. So my Sims possible genotypes and phenotypes are educated guesses of what I currently know in-game.

Carlston married Starr and they have five children with one on the way. They have four daughters and 1 son. From oldest to youngest: Carlie, Starrlah, Bumi, May and April. May is the only child, thus far, who inherited her grandmother’s hair color. It is likely Carlston has the Brown-black genotype as his dominant and recessive genes. May’s genotype is presumably Black-brown. Starr’s parents’ both have brown hair and her little brother also has brown hair. I think Starr has Brown-brown.

80% chance for brown hair
20% chance for black hair


60% chance for brown eyes
40% chance for green eyes


80% chance for a daughter
20% chance for a son


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Genesis married Angelica and they have five children with one on the way as well. They have four sons and one daughter. From oldest to youngest: Gene, Kin, Chancelor, Keenan and Angel. Some family members dye their hair but everyone’s original hair color is preset black. I have made it a habit to keep track of hair colors so it will not be an issue if the child takes a dyed hair color.

100% chance for black hair
Angelica’s parents both have preset black hair and her little brother too.
(Sidenote: I do not know which recessive gene Genesis carries but figure it is between brown or blonde. I will not possibly know until the children grow up and have children of their own who they may pass the recessive gene down to. Unless the parents' end up having a child with brown or blonde hair.)


60% chance for blue eyes
40% chance for green eyes


80% chance for a son
20% chance for a daughter


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Paige married Obi and they have 7 children. They have 4 sons and 3 daughters. From oldest to youngest: Reuben, Amelia + Leo (Fraternal Twins), Rebecca, Ace, Natalie and Jai.

57.1% chance for brown hair
42.9% chance for light golden brown hair


50% chance for green eyes
50% chance for light gray eyes
Jai has not aged into a toddler yet.


57.1% chance for a son
42.9% chance for a daughter


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Ryne has an ex-girlfriend named Raini and they have 3 children together, 2 sons and 1 daughter. From oldest to youngest: Deanna, DeMario and Harley.

33.3% chance for black hair
33.3% chance for blonde hair
33.3% chance for brown hair
(Of course these percentages may change if they were to have more children together. It would be possible for a light golden brown hair color as well.)


33.3% chance for light gray eyes
33.3% chance for blue eyes
33.3% chance for green eyes
(Of course these percentages may change if they were to have any more children together. Not likely though. But so far out of all the families I have, they have children with two different recessive hair colors.)


66.7% chance for a son
33.3% chance for a daughter


Ryne has a recessive hair color for black (his mother’s hair color) and Raini for blonde (randomly selected as she and her brother, Obi, are founders but that doesn’t mean Obi has a recessive trait for blonde hair too since siblings can carry different recessive genes). Ryne and Raini have two different recessive traits though it is said there must be two matching in order for a child to get a recessive hair color.

However, genetics are not so simple. I figure because heterozygous genotypes exist then this is what may happen as a possibility. Sometimes there are surprises in the genetic lottery such as this case. Genetics can pick and choose as long as there is a possibility for such an outcome. Of course not all outcomes may show up within one generation or possibly even the next. In this example, either black or blonde gets a chance in the inheritance pool. For the parents, black and blonde are recessive but for a child who inherits, one of those becomes their dominant while the other they become a carrier of to possibly pass down to their future child or grandchild. Maybe even further down the line of descendents.

I figure it is Ryne who a recessive trait for black hair because Hunter, his brother, has blonde hair that is a different root, base and highlight color than Harley’s. They only match in tips but nothing else. So I doubt this variation of blonde is Ryne’s parents’ recessive blonde, therefore must come from Raini. If these two in the future find a new romantic interest then it may be possible to further investigate their recessive hair colors with any other children they might procreate.

Hunter (Uncle)
R: 200, 185, 150
B: 225, 210, 170
H: 215, 205, 180
T: 200, 190, 155

Harley (Nephew)
R + H: 170, 150, 95
B + T: 200, 190, 155

Nathaniel and Trinity have 20 grandchildren with 2 on the way to add up to 22. Their oldest grandchild is Gene but no start yet on the 4th generation. But Chancelor has a promised wish to get married and both of his parents have a wish to see him get married. I am letting my Sims choose their own love interests and sometimes it takes awhile before they find someone they connect to. Or at least someone they thought they connected to. And sometimes they choose the same sex which is frustrating as a family-orientated player who loves playing with genetics. But I tend to have large families so the genetics will continue to play out in each generation.