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.

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.

Dominant and Recessive Genes
Dominant and recessive genes also play a part in The Sims 2 genetics. As quoted from AgifemGynor on TheSims2.com:

"For eye color and hair color: Each parent give one of his/her two alleles, chosen randomly, to the child. If the child has two dominant alleles, he will express one of them, chosen randomly. If he has two recessive alleles, he will also express one of them, chosen randomly. If he has a dominant and a recessive allele, the dominant allele will show, always.

Dominant alleles for hair color : black and brown. Recessive alleles for hair color : blonde and red. Dominant alleles for eyes color : brown and dark blue. Recessive alleles for eyes color : light blue, green and grey.

Alien eyes are dominant alleles. Custom content is either dominant or recessive, but the game chooses which it is, apparently randomly."

However, most custom content (especially custom skintones) is dominant.

Skin colors
As quoted from AgifemGynor on TheSims2.com:

"Skin colors work differently. The 4 skin tones are S1 (white), S2 (tanned), S3 (brown) and S4 (dark brown). They work on the same pattern of allele transmission, but the way the alleles are expressed depends on both. A person will have a skin tone chosen randomly between the tones of his alleles. A S2/S4 alleles person will have a skin tone of S2, S3 or S4, chosen randomly. S1, S2, S3 and S4 are equally recessive. Only alien skin tone seems to be dominant. Hence, a person with A/S1 will be green, always.

''CAS-created Sims, and apparently also generated adopted children, have homogeneous genes (2 brown alleles for a brown haired person for example), so we can always know their whole genome. NPCs (maids, pizza delivery person, etc.) and townies don't always have homogeneous genes, and they sometime dye their hair, so it's very hard, sometimes impossible, to know their genome. You can guess a part of it by having children with them, but a hidden recessive gene can remain hidden for generations, especially if the family has many dominant genes. Hair shape (curly, straight) is not a genetic trait."''

In practice, it appears that the alien skin tone is only effectively dominant over the S1 skin tone. A Sim who is A/S2, A/S3, or A/S4 may not be green, and may carry the allele for the alien skin tone as a recessive.

It has been noted that the mannequin skin tone, which can be accessed by using CAS debug mode, is also dominant.

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.

Continuity Errors with Genetics in the Sims
Two (dark) blue-eyed parents can have a brown-eyed child if one of the child's grandparents have brown eyes. Melody Tinker is an example of this. In real life, this is extremely rare. An individual carrying a brown-eye allele and a blue-eye allele is meant to express brown eyes (being the dominant trait); however if the blue-eye trait is expressed over the brown-eye trait then the individual may be mistaken as homozygous for blue-eyes (two blue-eye alleles), thus the individual's offspring is expected to have blue eyes too.

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. However custom eye color seem to be passed on a lot less likely in generation F1. (ex. Bright green, red). 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. Although a mutated eye color will always be one of the default colors, the same ways as most NPC's. (with the exception of those created prior to the start of the game.) Skin tones can be 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.

In addition, adopted children in The Sims 3, unlike in The Sims 2 seem to have a certain amount of their adoptive parents genomes fused into them, since their children can have exactly the same hair color or eye color as one of their grandparents who have a custom color.

Inheritable traits
Sims can inherit certain hidden traits from their parents. There is a 50% chance of passing one of these traits on if one parent has one.