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.
The Sims 2
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. But this isn't always the case, so two shy Sims most certainly can have an outgoing child and vice versa.
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 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."
Genetics aren't always equally dominant or recessive. For exampled, if a blonde-haired woman has a child with a red-haired man, it will usually have red hair, meaning that red hair must be slightly more dominant than blonde (which is true in real life). If a dark-skinned woman gets pregnant by a medium-skinned man, it will always have medium skin if it's a boy, and dark skin if it's a girl. If a brown-eyed woman gets pregnant by a dark blue-eyed man, their baby is somewhat more likely to have brown eyes than dark blue eyes (again, true in real life).
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.
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.
The Sims 3
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 blonde parents having a child with dark brown 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 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.
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.