Genetics

Genetics are the traits that a Sim has, such as the color of their hair, eyes, or skin. In Create-A-Sim, 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.

Genetics in The Sims 2 can also determine the personalities 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.

Dominant and Recessive Genes

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

For eye color and hair color: Each parent give one of his 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 choses which it is, apparently randomly.

Skin Colors

As quoted from AgifemGynor on The Sims 2 BBS:

Skin colors work differently. The 4 skin tones are S1 (white), S2 (tanned), S3 (brown) and S4 (dark brown). They work on the same patern 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 ton 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.

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.