Genetics

Genetics are traits such as hair color, eye color, or skin tone that Sims have, and can pass to their children. Genetics were introduced in The Sims 2; in The Sims, a baby that grew into a child received randomly selected body and head skins. in Create a Sim for The Sims 2 and The Sims 3, when 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.

Phenotypes

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

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, they will not be expressed in-game unless the Sim also expresses the alien skin tone.

Skin tone is passed in the same way, but the genes that are passed 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, one skin tone gene will be shown as "dominant" and one as "recessive", but the game itself does not treat any normal skin tone as dominant.

The alien skin tone appears to be a special case. 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 the NPC Pollination Technician. It is possible for the child of an alien-Sim hybrid to receive and carry the alien-skin gene but express a normal 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 a 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. Different elements of a facial feature, such as eye size and eye shape, are inherited separately.

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 either the 2nd template or the 26th template. Sims created using these templates can pass this "brokenness" on to their descendants.

First born effect
The Sims 2 is subject to what is known as the "first born effect", 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.

Physical characteristics
Genetics in The Sims 3 are much 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 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 hue (light or dark) and general color 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.