breed animals with desired phenotypical traits/ no health issues together via natural selection but with human interception. example: if a dogs family has a dominant allele of being aggressive and all puppies within that litter and previous generations tend to be protective, they may be bred with a muscly dog to achieve a dog with the same dominant aggression allele but is also very strong for a guard-dog- can use a punnett square to theoretically predict % chance of puppies having the desired characteristic.