Outcrossing

Outcrossing is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of all individuals being subject to disease or reducing genetic abnormalities (only within the first generation). It actually can serve to increase the number of individuals who carry a disease recessively.

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 "in-breeding" which is the production of offspring by parents more closely related than the average.

Outcrossing is now the norm of most purposeful 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 recessives, outcrossing allows for the recessive traits to migrate across a population. It may actually increase the number of individuals carrying a disease. The outcrossing breeder then may have individuals that have many deleterious genes that are expressed by placing their animals against a similarly outcrossed individual. There is now a gamut of deleterious genes within each individual in many breeds. However one may increase the variance of genes within the gene pool by outcrossing, protecting against extinction by a single stressor from the environment. In cats, there is currently a study running to determine the genetic diversity within the cat breeds.

Outcrossing is believed to be the "norm" in the wild. However, it is not logical as migration occurs by necessity. Feral cats, for example are one of the most inbred as individuals remain nearby their original homes, unless environmental stresses drive them to migration.

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 is accomplished by breeding two individuals of known genetic status, usually they are related.

In nature, where breeding is not managed, outcrossing rates may be estimated by genetic analysis, by employing mathematical models of mating systems such as the mixed mating model or the effective selfing model. This allows calculation of the amount of genetic exchange between populations, and thus provides insights into the biogeography and phytogeography of species.

Gregor Mendel used outcrossing in his experiments with flowers for his breeding stock. He then used the resulting offspring to chart inheritance patterns, using the crossing of siblings, and backcrossing to parents to determine how inheritance functioned.