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.
Generating congenic strains
Congenic strains are generated in the laboratory by mating two inbred strains (usually rats or mice), and backcrossing the descendants 5-10 generations with one of the original strains, known as the recipient strain. Typically selection for either phenotype or genotype is performed prior to each backcross generation. In this manner either an interesting phenotype, or a defined chromosomal region assayed by genotype is passed from the donor strain onto an otherwise uniform recipient background. Congenic mice/rats can then be compared to the pure recipient strain to determine whether they are phenotypically different if selection was for a genotypic region, or to identify the critical genetic locus, if selection was for a phenotype.
Speed congenics can be produced in as little as 5 back cross generations, through the selection at each generation of offspring that not only retain the desired chromosomal fragment, but that also 'lose' the maximum amount of background genetic information from the donor strain. This is also known as 'Marker Assisted' congenics, due to the use of genetic markers, typically microsatellite markers, but now, more commonly, single nucleotide polymorphism markers (SNPs). The process can be further aided by the superovulation of females, to produce many more eggs.
Notes and references
- "Congenic - definition from Biology-Online.org". Biology-Online.org dictionary. Biology-Online.org. Retrieved 2006-09-25.
- Markel P, Shu P, Ebeling C et al. (November 1997). "Theoretical and empirical issues for marker-assisted breeding of congenic mouse strains". Nat. Genet. 17 (3): 280–284. doi:10.1038/ng1197-280. PMID 9354790.
Congenic strains are discussed in detail in Lee Silver's online book Mouse Genetics: Concepts and Applications:
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