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Original Poster
#1 Old 30th Jul 2021 at 7:46 PM
Default 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.)

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


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


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


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.

Because I'm Fragile...but I'm not "that" fragile.
Original Poster
#2 Old 12th Aug 2021 at 5:58 AM
Case Study 2: Different Levels of Melanin

The Yamazaki Family
(Sidenote: Kiba uses a non-default skin and because of the lightness of it, it may affect any chances for incomplete dominance or this couple just doesn't have a high percentage chance for incomplete dominance for skin color in their genes like Nathaniel and Trinity.)
(Sidenote 2: Kiba and Simone are founders and created in CAS while Yoshi and Naomi were created as teens using the Play with Genetics feature from Kiba and Simone. The other four children were born in-game.)

Kiba and Simone are married with 6 children. They have 4 sons and 2 daughters. One of their daughters, Megumi has a lighter shade of black (60, 60, 60), close to jet black, than her father’s pure black hair (0, 0, 0).

Megumi’s Approximate Hair Color Compared to Her Father’s

These instances of the same hair color but varying in lightness or darkness may be associated with imitating different melanin levels of eumelanin and pheomelanin.

Eumelanin is responsible for creating black or brown color. If there is little of this melanin present in an individual then their hair color will be light and can consist of many color variations of blonde. If there is a lot of this melanin present then the hair color will be dark and consist of many color variations of brown or black.

Pheomelanin is responsible for creating red or orange color. Those with orange or red hair can have varying amounts of eumelanin which results in different hair colors such as auburn or strawberry blonde.

As mentioned before, like skin color, hair color is a polygenic trait.


This is an old CAS Play with Genetics Test.

Mother ‘s Hair Color
R: 48, 38, 28
B: 97, 56, 23
H: 120, 74, 43
T: 74, 48, 30

Father’s Hair Color
R: 40, 40, 40
B: 65, 65, 65
H: 36, 36, 36
T: 62, 62, 62

Child 11’s Hair Color
R + H + T: 150, 110, 75
B: 150, 95, 50

Because his hair is orange and his mother's is brown and his father's is black then this must be a recessive hair color of one of them.

Child 3 has a different variation of black than her father's
R + H + T: 40, 40, 40
B: 70, 70, 70

Child 10 seems to have a darker color of a recessive blonde based on the root color from her siblings with blonde hair. So melanin levels can also affect recessive hair colors too in TS3. Fascinating. Also, because child 10 has a lighter black root hair color than her father's but a darker blonde recessive base hair color then technically that's an example of codominance besides differing melanin levels in recessive hair colors. So one parent has blonde and the other has orange recessive hair colors.
R: 60, 60, 60
B: 70, 60, 40
H + T: 50, 40, 20


Another Play with Genetics Test

R: 123, 67, 19
B: 171, 93, 35
H: 190, 116, 58
T: 194, 119, 56

R: 48, 38, 28
B: 97, 56, 23
H: 120, 74, 43
T: 74, 48, 30



The children’s hair color is:
R + H + T: 150, 110, 75
B: 150, 95, 50

One of the parents has black as a recessive trait and I think purple eyes is a mutation? Though I read that purple eyes do exist but rare based on lack of melanin in the iris and how light is reflecting from the blood vessels in the eyes which gives off a purple hue. If I even remember that correctly. I guess it may be associated with albinism but don't really know. I am starting to wonder if a mutation chance is higher or lower in some Sims. Perhaps there is a lot of information that gets randomized once a Sim is created. How much hidden Sim Data could one Sim have?

During other test and examples of families living in the Sim world, some Sims do not create children with varying lighter or darker colors, as in, strictly either the mother’s or father’s hair or skin color exactly. However, other Sims it seems, have genes showcasing a chance for incomplete dominance and a chance for lighter or darker versions of the parent's hair color or recessive hair color(s).

Are there hidden hair color presets that are rare to see or are these actually being calculated in some way from a hidden gameplay mechanic? Maybe a bit of both? Either way, it seems to happen under certain conditions in order to see these types of unique hair colors. I suspect there may be more of those.

Does hair color blending exist in TS3 but just rare depending on the coupling and genes? It definitely seems that way. Like a box of chocolates, you never know what you are going to get but at the same time makes sense and follows something based on reality. I am going to take an educated guess the genetic system is based on more than just the principles of inheritance by Gregor Mendel.

“Our modern understanding of how traits may be inherited through generations comes from the principles proposed by Gregor Mendel in 1865. However, Mendel didn't discover these foundational principles of inheritance by studying human beings, but rather by studying Pisum sativum, or the common pea plant.”

“Today, whether you are talking about pea plants or human beings, genetic traits that follow the rules of inheritance that Mendel proposed are called Mendelian.”


Because I'm Fragile...but I'm not "that" fragile.
Original Poster
#3 Old 28th Oct 2021 at 5:05 PM
Case Study 3: Breaking the Family Tree

The Rumbaa Family – Genesis Rumbaa - 2nd Generation

Genesis and his wife Angelica live in Roaring Heights. Nathaniel and Trinity, Genesis’s parents’ live in Moonlight Falls while Francesco and Alexandria, Angelica’s parents’ and younger brother Keanu are currently in the library waiting for the next round to be moved into Roaring Heights. Currently they are both disconnected from the 1st Generation. When the family tree is intact and all are in the same neighborhood I know there is a chance for a grandchild to inherit a grandparent’s hair/eye color, but what happens when the grandparents are not present because you move the 2nd Generation to a different neighborhood, thus breaking the family tree?

There are two methods which lead to the same results.

Method 1: CAH (Create-a-Household)

I used NRass to take Genesis and Angelica into Stylist to save them to the CAS bin then exited out to enter CAH in Edit Town. Once in CAH I opened up the saved Sims bin to retrieve the two Sims and then proceeded to Play with Genetics. The children came out with black being the most common, blonde and orange hair. There was no preset brown among the genetic calculator which is Genesis’s father hair color.
So to answer the question I posed to better understand the implications of my chosen playstyle, the answer is: The game appears to be coded in a way that grandparents must be present in the same neighborhood in order for there to be a possibility of a grandchild to inherit their hair/eye color. The grandparent’s dominant genes become non-existent and no longer included in the gene pool with any family unit who is disconnected.

Method 2: Instant Baby (NRass Master Controller)

As mentioned before, this replicates the same results as method 1 and this can be taxing and crash the game. I tend to get more stable results when I keep the game on pause when I do this. However, I would not recommend this method.

Recessive Genes

To refresh, recessive genes are those that one is a carrier of but does not have, but can pass the gene(s) down to their child either as a carrier or inheritor.
The grandparents’ hair/eye color will not have a chance to get passed down to descendants when the family tree becomes incomplete, however, their recessive genes do which seem to imply the game is coded to specifically use the hidden assigned recessive gene(s) in addition to inheriting grandparents’ hair/eye color when the family tree is intact. Otherwise, when the family tree becomes fragmentary, only the recessive genes will continue through the generations regardless of the condition of the family tree.

The 1st Generation Recessive Hair Colors

Trinity and Nathaniel produced children with Black, Blonde, Brown and Orange hair. Francesco and Alexandria produced children with Black, Blonde and Orange hair. In order to attempt to get the Punnett Square accurate, I have to use trial and error based on in-game data, but there is still quite a bit of guesswork. Meaning I know which couple has which recessive hair/eye colors, be that as it may, there is no known way of pinpointing which recessive hair/eye color is whose. All I know for certain, is both Blonde and Orange are the known recessive hair colors that can be passed down to the 2nd generation and so on.

Certain genes get turned on or expressed while the rest is turned off or unexpressed. Basically when the genetic calculator chooses between the father’s or mother’s or grandfather’s or grandmother’s or recessive hair/eye color for the offspring. So many possibilities can carry different genetic information that can result in similar looking siblings to siblings who look nothing like each other to descendants with an unexpressed hair/eye color from a previous generation.

The Possible Genotypes and Phenotypes of Genesis and Angelica

(Now I know the game is not coded to count the grandparents hair/eye color as recessive, but will have the extra genetic information to choose from when considering the phenotype of connected families. I will have to revise the Punnett Squares to reflect this new discovery.)

(However, since I cannot figure out for certain which recessive gene is whose in some or most cases, then I will carry on in a different approach without the Punnett Square. For example, when one or both Sim(s) is/are known to have more than one recessive gene for hair/eye color then it gets especially complicated not being able to know for certain which Sim has what. It would have been nice to have options, thus control over recessive genes being chosen by the player using CASt to pick or create any color besides an option to choose randomly from the CAS hidden presets. Also, that up to 3 different variations of the same hair/eye color could also be incorporated such as one parent having 3 different shades of blonde hair and 2 different shades green eyes or none at all and another parent having different 2 different shades of brown hair and 3 different shades of grey eyes. And of course, on top of that, up to 3 different recessive hair/eye colors. That seems to be the case in TS3 but just not player controllable unfortunately.)

Because I'm Fragile...but I'm not "that" fragile.
#4 Old 28th Oct 2021 at 5:29 PM
I know you're looking into hair and eyes but it's striking how persistent grandma's jawline & cheekbones are.
Original Poster
#5 Old 29th Oct 2021 at 8:50 PM
Yes, it is interesting to see which facial features and body types get inherited through the generations as well. Genesis's children mostly take after their mother's facial features. There are so far two out of six of their children who inherited a more rounded face.

Angel (Natural hair color is preset black)
-Mother's eye shape
-Father's eye color
-Mother's nose
-Mother's lip shape
-Closer to her father's skin tone
-Father's rounded face
-Curvier physique than her mother (Though lately I notice she autonomously will work out so her weight changes a bit.)
-Smaller breast than her mother but that may change when she ages up

Dimitri (Currently a toddler but aged up to see what he would look like.)
-Mother's eye shape
-Father's eye color
-Father's nose
-Father's lip shape
-Closer to his mother's skin tone
-A slight mix between father's round face and mother's angular face

There are some really unique mixtures of parents genetics but I do not see those type of genetic calculations as often as I would like.

I suspect just like recessive hair/eye colors that sometimes these kinds of combinations can be rare to come across. Especially, if one of the parent's facial features and/or body type is more prominent. Perhaps if both parents' genetics are balanced out with each other then maybe more intermixing may take place in their offspring.

Because I'm Fragile...but I'm not "that" fragile.
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