Inbreeding is the mating of closely related individuals that share common ancestry. Inbreeding is used in animal husbandry to retain desirable characteristics and eliminate undesirable ones. However, the mating of siblings in farm animals can only be continued for a few generations as viability and fertility are adversely affected. Inbreeding can cause harmful genes that are recessive in both parents to be manifest in the offspring. Linebreeding is a mechanism used by agricultural breeders to concentrate the genes of a certain ancestor in a strain of animals. This is accomplished by mating a female with her grandsire or uncle, for example. The probability of undesirable genes in the offspring is reduced.

There are some types of inbreeding which occur in nature. Self-fertilization, for example, occurs in bisexual organisms such as most flowering plants, some protozoans and invertebrates. It is the fusion of male and female gametes produced by the same individual. As an evolutionary and reproductive mechanism, self-fertilization allows an isolated individual to create a local population. However, without a degree of variability among the individuals produced, possibilities for adaptation to environmental change are reduced.

Outbreeding is defined as mating individuals that are not related at all. Some cross-species outbreeding has occurred, particularly between the horse and the ass to produce the mule.

Crossbreeding has been practiced for a long time and refers to mating within the same species. The main function of crossbreeding is to produce offspring with carry the desirable traits of both parents.