Outcrossing or outbreeding is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of an individual being subject to disease or reducing genetic abnormalities.
It is used in line breeding to restore vigor or size and fertility to a breeding line. "Linebreeding" is where animals carry a common ancestor in their pedigrees and are bred together, should be considered distinct from the term "inbreeding" which is the production of offspring by parents more closely related than the average.
Outcrossing is now the norm of most purposeful animal breeding, contrary to what is commonly believed. The outcrossing breeder intends to remove the traits by using "new blood". With dominant traits, one can still see the expression of the traits and can remove those traits whether one outcrosses, line breeds or inbreds. With recessive traits, outcrossing allows for the recessive traits to migrate across a population. The outcrossing breeder then may have individuals that have many deleterious genes that may be expressed by subsequent inbreeding. There is now a gamut of deleterious genes within each individual in many dog breeds.
Increasing the variation of genes or alleles within the gene pool may protect against extinction by stressors from the environment. For example, in this context, a recent veterinary medicine study tried to determine the genetic diversity within cat breeds.
Outcrossing is believed to be the "norm" in the wild. However, this is not necessarily the case. Feral cats, for example are one of the most inbred as individuals remain nearby their original homes, unless environmental stresses drive them to migration. Outcrossing in plants is usually enforced by self-incompatibility.
Breeders inbreed within their genetic pool, attempting to maintain desirable traits and to cull those traits that are undesirable. When undesirable traits begin to appear, mates are selected to determine if a trait is recessive or dominant. Removal of the trait is accomplished by breeding two individuals known not to carry it.
Gregor Mendel used outcrossing in his experiments with flowers. He then used the resulting offspring to chart inheritance patterns, using the crossing of siblings, and backcrossing to parents to determine how inheritance functioned.