Genetics

Genetics are traits such as hair color, eye color, or skin tone that Sims have, and can pass to their children. In The Sims, genetics did not exist, and a baby that grew into a child received a randomly selected body and head skin. Genetics were officially introduced in The Sims 2. In Create a Sim for The Sims 2, The Sims 3, and The Sims 4, once the player has made one male Sim and one female Sim, they can create a child for the couple that has the genetics of both parents. In The Sims 4, players can also create a genetic child for Sims of the same gender.

The Sims 2

Phenotypes

;Sim eye colors

Eye colors	Phenotypes
Dominant eye colors
Recessive eye colors

Pet eye colors

Species	Phenotypes
Cat
Dog

Hair colors	Phenotypes
Dominant hair colors
Recessive hair colors

Skintones	Phenotypes
Dominant skintones
Co-dominant skintones

For eye color, hair color, and skin tone, each parent has two genes. When a baby's genetics are determined, each parent gives a randomly determined gene for eye color, hair color, and skin tone.

When viewing a Sim's DNA in SimPE, one set of genes will be shown as dominant, and one as recessive, but this may or may not reflect whether the game treats a gene as dominant or recessive. The genes are shown as alphanumeric strings, as follows:

;Sim eye color genes

Eye color	Code
Brown	32dee745-b6ce-419f-9e86-ae93802d2682
Dark Blue	2d6839c5-0b7c-48a1-9c55-4bd9cc873b0f
Green	51c4a750-c9f4-4cfe-801c-898efc360cb7
Light Blue	e43f3360-3a08-4755-8b83-a0d37a6c424b
Grey	0758508c-7111-40f9-b33b-706464626ac9
Alien	12d4f3e1-fdbe-4fe7-ace3-46dd9ff52b51

Cat eye color genes

Eye color	Code
Dark Blue	314fb926-ee6e-4fb8-94f0-3f67e4c7443d
Dark Brown	0625b1bc-8363-48ec-b7af-46aa6dd6273e
Dark Green	22a77e4a-1fc7-4098-b4e0-ddd29bb38ac5
Gray	6f35a34d-6a92-4776-b0fc-c51bcf268e95
Light Blue	b9bf8eb8-0b7b-4051-8e14-d36f362e9108
Light Green	8de4d9a1-c401-4893-9a6c-588ab671ac0d
Gold	7bab7135-9927-4a19-80d3-cc408da053d4
Light Brown	b178af36-ae7c-4930-b83b-876418e346f4

Dog eye color genes

Eye color	Code
Purple	480af2ee-5f4f-4bf5-b0ca-f615b6dee919
Gray	b927db4c-b9e5-44e7-acde-a79bbdffde3a
Red	c4671cb6-d305-4701-bdb8-c99d5319de3f
Dark Brown	011d79f7-afb2-41c5-a09a-158f31dbbf70
Light Blue	629e24da-6c86-4e4e-806d-4e55f941a043
Light Brown	6a175725-c3db-40f3-9391-9ca193f8562f
Dark Blue	cb37d54b-3458-4ac8-be5f-add84d129f93
Dark Green	cf6c7824-e3e4-43ef-8542-053e59f5ae57
Light Green	bfca3600-e5b6-41ac-944a-165453eb50fa
Gold	f5f89e13-9de8-4f61-abb7-2c8a31757a70

Hair color	Code
Black	00000001-0000-0000-0000-000000000000
Brown	00000002-0000-0000-0000-000000000000
Blond	00000003-0000-0000-0000-000000000000
Red	00000004-0000-0000-0000-000000000000	Gray	00000005-0000-0000-0000-000000000000

Skintone	Code
Light (Skintone 1)	00000001-0000-0000-0000-000000000000
Tan (Skintone 2)	00000002-0000-0000-0000-000000000000
Medium (Skintone 3)	00000003-0000-0000-0000-000000000000
Dark (Skintone 4)	00000004-0000-0000-0000-000000000000
Alien	6baf064a-85ad-4e37-8d81-a987e9f8da46
Mannequin	b9a94827-7544-450c-a8f4-6f643ae89a71
Zombie	b6ee1dbc-5bb3-4146-8315-02bd64eda707

The game considers certain eye colors and hair colors as dominant and some as recessive. If a baby gets a dominant and recessive gene for eye color or hair color, the dominant color will always be visible. However, if the baby gets two dominant (or recessive) genes for eye color or hair color, one will be randomly selected as "dominant". That color will be visible, and that gene will show as "dominant" in SimPE. The other will be carried as a recessive and will be shown as such in SimPE. It should be noted that, while alien eyes are dominant, the alien eyes will look identical to brown ones, should the Sim not also express the alien skin tone.

Skin tone is passed in the same way. However, if a Sim receives two genes for normal Maxis skin tones, those genes define the ends of a range in which 1 is light skin, 2 is tan, 3 is medium, and 4 is dark. Within that range, all skin tones are equally likely, and which one the baby has is determined at random. For example, a baby who gets a gene for tan skin and one for dark skin may have tan, medium, or dark skin. In SimPE, these genes will be shown as "SkintoneRange". While one will be shown as "dominant" and one as "recessive", the game itself does not treat any normal skin tone as dominant.

The alien skin tone appears to be a special case. Like custom skin tones, it is outside the 1-4 range, and is generally treated as dominant, but how this is expressed depends on other conditions. The alien skin tone does not appear to be truly dominant unless one parent is homozygous for it. If neither parent is homozygous for the alien skin tone, and the baby gets both that gene and a gene for a normal Maxis skin tone, the baby may express either gene while carrying the other as a recessive. So, it's possible for the child of an alien-Sim hybrid to receive and carry the alien-skin gene while expressing a normal skin tone and then have a green-skinned child.

Additionally, for cats and dogs created in CAS with hetero-chromatic (two different-colored) eyes, only the phenotype displayed in the pet's right eye will be present in their genetic code.

Custom phenotypes

The Sims 2 allows the creation of custom hair, eyes, and skin tones, and many are available for download. These custom items are usually treated as dominant, and they have additional effects on genetics.

Hair

How custom hair behaves genetically depends on whether or not it has been placed in one of the Maxis color bins. Custom hair that has been properly "binned" as black, brown, blond, or red behaves the same as Maxis hair, though it will still have the custom-content "star" in CAS and change appearance. Like Maxis hair, it may be randomly assigned to a Sim when his or her genetics are determined. Each custom hairstyle that has not been "binned" has its own genetic code. A Sim who inherits a custom-hair gene from a parent of the same gender will receive that style. However, a Sim will not have custom-hair genetics unless he/she inherited them, or was given hair from the Custom bin when created in CAS, as Change Appearance does not change genetics.

Eyes

Eye dominance is on a scale of 0 to 4. Maxis-dominant is at 1, and Maxis-recessive is at 2; values greater than 2 are usable but unused. By default, custom eyes are at 0, which makes them "super-dominant" overall Maxis eyes. However, some content creators may "geneticize" eyes by editing this value.

Skintones

By default, custom skin tones are outside the normal range of skin tones. Receiving a gene for a custom skin tone, or for the alien skin tone or the hidden mannequin skin tone, changes the way skin tones are handled. If a Sim receives a gene for a custom skin tone, the skin tone range is ignored, and the skin tone is treated as a dominant/recessive trait. Custom skin tones are dominant, but how this is expressed in-game may depend on whether the parent who passed the custom-skin gene was homozygous for that tone.^{[confirmation needed]}

Some content creators may "geneticize" custom skin tones. This is done by editing a skin tone's genetic value so that it falls within the normal Maxis range of 0.01 to 0.99. When determining a baby's skin tone, "geneticized" custom tones are treated the same as Maxis tones. Since the S1, S2, S3, and S4 tones have genetic values of 0.1, 0.3, 0.6, and 0.9, it is possible to have "geneticized" skin tones that fall between two Maxis skin tones. So, for example, if a custom skin tone that's "geneticized" with a value of 0.45 is installed, a baby born to parents with the Maxis S2 and S3 skin tones may have that skin tone.

Personality

Genetics in The Sims 2 can also determine the personality of a Sim. If both of the parents of a child each have four nice points, for example, it is likely that the child will also have four nice points. However, this isn't always the case; two shy Sims most certainly can have an outgoing child and vice versa. The point assignment comes from the mother, father, or completely at random.

A born-in-game Sim can have between 25 and 35 personality points, whereas a Create-A-Sim Sim can only have 25. Personality points are assigned to each trait in random order. If the number of points at any time reaches 35, then every trait being set thereafter will have no points in it. On the other hand, if, after setting all five traits, the point total is less than 25, then points will randomly be assigned to any traits with low numbers until the point total equals 25.

Spawned PlantSims will always have the same personality as their parent, though they will have different zodiac signs.

Facial features

Facial features are also genetically reproduced from parent to child in the game. For example, if a parent has large eyes, the child may have the same large eyes. Facial features do not, however, cancel each other out - for example, if one parent had a large nose and the other parent had a small nose, the child will not automatically have a medium-sized nose - they will have either the large nose or the small nose.

Errors

Broken Maxis templates

Two of the default CAS face templates, the 21st and 25th, are not linked correctly to other ages. Instead, they are linked to the 2nd template. Sims created using these templates can pass this "brokenness" on to their descendants. There's a mod that can fix this, but it will not affect existing Sims who use those templates.

First born effect

The Sims 2 is subject to what is known as the "first born effect"/ "first born syndrome", in which all single children born in a family have the same genetics. This happens because the randomizer which affects Sim genetics always starts at the same point each time the game is started. The "first born effect" can be mitigated during a game session by going into CAS and clicking on the "Randomize" button several times, as this seems to "shake up" the randomizer. It is not necessary to save the random Sims that are generated. The Lot Debugger, or "Batbox", available from More Awesome Than You, has a feature that essentially automates this process.

Notes

Sims created in Create a Sim will have homogeneous genes, unless they were created with "Make a Child". Sims generated by the game, such as replacement townies and generated adoptees also have homogeneous genes. Pre-made townies and NPCs may or may not have homogeneous genes, and often have visible characteristics, such as hair color, that is different from their genetics.

"Make a Child" will only mix the genetics of two adult Sims. While elder males can impregnate adult females in-game, "Make a Child" does not recognize this.

The grey hair bin used by elders is not a genetic bin. It does have a bin code, 00000005-0000-0000-0000-000000000000, but that code is not used in Sim DNA.

The Sims 3

Physical characteristics

Genetics in The Sims 3 are much more simplified. There are no dominant and recessive genes, and the game simply combines features of the parents, pulling occasionally from earlier generations such as grandparents. There is a very high mutation (i.e. two dark-haired parents having a child with blonde hair, even though nobody in their family has that hair color) rate of 10%, which can be lowered to a more reasonable rate using mods.

Skin tones are blended by The Sims 3 genetics. For example, a Sim with light green skin and another with dark blue can still have a child with dark green or light blue skin, as the light or dark and hue are handed down separately.

Inheritable traits

Sims may pass on traits when traits are randomly determined. Sims can also inherit hidden traits from their parents; there is a 50% chance of passing one of these traits on if one of the parents has one.

The Sims 4

Physical characteristics

The Sims 4 shows a wide variety of mixed features. The children tend to come out with features ranging from exactly like the one parent to an in-between mix. It appears that there are no recessive genes in The Sims 4 that are passed down. This makes it impossible to inherit a gene from a grandparent or any other ancestor unless one of the parents displays the trait.

Skin tone

In The Sims 4, skin color is usually a mix of the parents. If one parent has a dark skin tone and the other has a light skin tone, the offspring will almost always end up in the middle; in such a case of a light-skinned parent and a dark-skinned parent, it is very rare for the child to have the exact same skin tone as either parent. If one parent has an unnatural skin tone such as blue or green, their offspring will always have either one parent's skin tone or the other's.

Hair

Hair color tends to come out as one of the parents. If one parent has brown hair and the other blonde, the offspring will have a 50% chance of having either. When a Sim with an unnatural hair color such as blue or pink has a child with a Sim that has a natural hair color, the hair color of the child may not be the same as either of the parents. The same applies to hair texture; however, if a Sim inherits their hair texture from one parent, they will also inherit its hair color as well. For example, if one parent has straight blonde hair, and the other parent has afro-textured black hair, the offspring can only either have straight blonde hair, or afro-textured black hair.

Eyes

Eye color usually comes out as one of the parents, but the five eye colors added in Patch 5 seem to be a special case. All of them share a flag value with one of the eye colors from the unpatched game; olive green hazel shares the same value as light green, dark blue hazel shares the same value as blue hazel, dark blue shares the same value as grey, grey-brown hazel shares the same value as light brown, and purple shares the same value as amber. Because of this, if one parent has dark blue eyes, for example, if their offspring inherits that parent's eye color, it has a 50% chance of having grey eyes, even if no one else in their family has grey eyes.

Gallery

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  The genetics of two adult Sims in The Sims 2.

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  An example of genetics in The Sims 2.

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  An example of the "Play with Genetics" feature in The Sims 3.

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  An example of genetics in The Sims 4.