For most practical purposes, an inbred strain can be regarded as an immortal clone of genetically identical individuals. According to the International Committee on the Genetic Nomenclature of the Mouse:

A strain shall be regarded as inbred when it has been brother x sister mated (hereafter called b x s) for 20 or more consecutive generations (F20), and can be traced back to a single ancestral breeding pair in the 20th or subsequent generations. Parent x offspring matings may be substituted for b x s matings provided that, in the case of consecutive parent x offspring matings, the mating in each case is to the younger of the two parents.

Similar definitions apply to other species.

Brother x sister mating increases homozygosity at each gene locus at about 19.1% per generation. So a strain is never fully inbred, but homozygosity exceeds 98% after 20 generations. Eventually, an equilibrium is set up between the loss of heterozygosity due to continued inbreeding, and the generation of new genetic variation by mutation. A single breeding pair in the 20th or a subsequent generation is chosen to ensure that the strain is also isogenic, i.e. all individuals are genetically identical (see below). Where parent x offspring matings are necessary, the younger of the two parents is used to avoid several generations of backcrossing to a single individual, which is not genetically equivalent to brother x sister mating.

Most of the common inbred strains of mice and rats were developed between 1906 and about 1940, though a few strains continue to be developed each year. For example, the most widely used strains of mice are C57BL, C3H, CBA, BALB/c, and AKR, which were all developed between 1920 and 1936. Several strains (including C3H and AKR) were selected for a high incidence of neoplasia. Currently there are over 400 inbred mouse and 200 inbred rat strains (many more if congenic and recombinant inbred strains are also counted), but only a handful of inbred strains of guinea pigs, hamsters, gerbils, chickens and rabbits.

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