What is "congenic"

a. (context genetics English) (''of different strains of an organism'') That are genetically identical except for a variation in a limited genetic region or locus.

In genetics, two organisms that differ in only one locus and a linked segment of chromosome are defined as congenic. Similarly, organisms that are coisogenic differ in one locus only and not in the surrounding chromosome. Unlike congenic organisms, coisogenic organisms cannot be bred and only occur through spontaneous or targeted mutation at the locus.