Various mating schemes of animals are classified under two broad categories -- inbreeding and outbreeding. Classification depends on the closeness of the biological relationship between mates. Within each category, a wide variation in intensity of this relationship exists. A very fine line separates the two categories. Mating closely related animals (for example, parent and offspring, full brother and sister or half brother and sister) is inbreeding. With less closely related animals (first cousins, second cousins), people disagree about where to draw the line between inbreeding and outbreeding.
Technically, inbreeding is defined as the mating of animals more closely related than the average relationship within the breed or population concerned. Matings between animals less closely related than this, then, would constitute outbreeding. These two systems of mating, with varying intensities in each, are described in Table 1. Matings indicated within the inbreeding category are self-explanatory; those within the outbreeding category are defined in the glossary.
Degrees of inbreeding and outbreeding arranged according to biological relationship between indicated mates. (In reading from top to bottom, biological relationship between mates steadily decreases.)
Inbreeding or outbreeding Coefficient of relationship between mates Biological relationship between mates
Inbreeding 50 percent Parent x offspring; full sibs
Inbreeding 25 percent Half-sibs; double 1st cousins; aunt x nephew; uncle x niece
Inbreeding 12-1/2 percent First cousins
Inbreeding 6-1/4 percent Second cousins
Inbreeding ? Linebreeding1
Inbreeding/outbreeding 0 percent Random mating within breed or population2
Outbreeding 0 percent Outcrossing
Outbreeding 0 percent Breed crossing
Outbreeding 0 percent Species crossing
Outbreeding 0 percent Genus crossing
1In a linebreeding program, the coefficient of relationship between mates is usually low; however, it can be quite variable.
2Random mating within a breed or population means that mates are chosen by chance. It should be understood that under this circumstance it is possible that either inbreeding or outbreeding could occur.
Biological relationships between animals
Individuals are considered to be biologically related when they have one or more common ancestors. For practical purposes, if two individuals have no common ancestor within the last five or six generations, they are considered unrelated.
Biological relationship is important in animal breeding because the closer the relationship, the higher the percentage of like genes the two individuals carry. Closeness of relationship is determined by three factors:
How far back in the two animals' pedigrees the common ancestor appears
How many common ancestors they have
How frequently the common ancestors appear. It is also influenced by any inbreeding of the common ancestor or ancestors