Definition

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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