Congenic

In genetics, two organisms that differ in one locus are defined as congenic or coisogenic.
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Animals which have been bred to be genetically identical except for a single gene locus. This is achieved by superimposing the locus of interest on the genetic background of another by first crossing two inbred lines followed by extensive (about 20 generations) backcrossing hybrids to one parental line (the background strain) while selecting for the alleles of the locus of interest of the other. The result is an inbred strain uniquely identified by a difference at a single locus.