Just because scientists did not know exactly how heredity and genetics worked did not stop people from using what they observed. People have been manipulating the phenotypes of plants and animals for centuries. This practice is called selective breeding.

Farmers, for example, might selectively breed plants that are more resistant to disease or that yield a larger crop. Dairy farmers might selectively breed dairy cows that routinely produce more milk. Poultry farmers might selectively breed hens that lay the most eggs. If a dairy farmer crosses a cow that produces a lot of milk with a bull that comes from a line of long-lived cattle, the resulting offspring could carry both of these advantageous traits. The offspring is called a hybrid. Animal and plant hybrids are produced by crossing parents that are genetically different.

If this selective breeding goes on from generation to generation, there is a danger of losing genetic diversity. Genetic diversity is important because it allows organisms to adapt to adverse conditions such as a lack of food or outbreaks of disease. When animals or plants that have a desired trait are bred with another that has the same trait, other genes (the ones that do not code for the advantageous trait being selected for, but may code for other important traits) are in danger of getting permanently lost. This is called in-breeding depression. For this reason, breeders try very carefully to produce animals and plants that are heterozygous, in order to preserve genetic diversity.