Genetics mean what the kids will look like Ex. brown and brown make brown and blonde and blonde make blonde.

But sometimes there is a rare child that looks like neither of the parents.

In Virtual Families 1 they use genetics and rare children

In Virtual Families 2 it is always a random child.

In Virtual Villagers series it is genetics and rare children

In Fish Tycoon there is a fish that makes rare children appear more often.