A strain that differs from another inbred strain by a single mutant gene.
Coisogenic strains arise from a spontaneous mutation.
Offspring that carry the differential gene are then mated to another individual of the inbred strain. This backcrossing is carried out for at least 10 generations to produce a congenic strain. The first generation product of a mating between two different inbred strains. Such animals are often referred to as outbred and random bred.