Source:http://linkedlifedata.com/resource/umls/id/C1516786
NCI: Congenic strains are produced by transferring a mutation from one genetic background to a specific inbred strain through repeated backcrossing. The congenic strain and the inbred partner are expected to be identical at all loci except for the transferred locus and a linked segment of chromosome. The size of the segment and the possibility of transferred alleles on other chromosomes depend on the number of backcross generations. A strain is considered fully congenic after ten generations of backcrossing (N10). (from JAX Strain Information)