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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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 the personality (but different Zodiac signs) as their parent.

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. For example, a child may have his/her mother's eye shape, but the father's eye size.

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.