Considerations for choosing controls
Controls for Genetically Engineered Mice
If a mutation is maintained on a standard inbred background, that inbred strain is the appropriate control. If a mutation is maintained on either a mixed or segregating genetic background, wild-type mice (phenotypically non–mutant mice) from the colony are the best controls. However, because many of our genetically engineered strains are maintained by homozygous matings, wild-type controls from the colony are not available. F2 hybrids are often used as approximate physiological controls for strains carrying targeted mutations (knockouts) maintained on a mixed C57BL/6 x 129 background (designated B6;129). The F2 generation is produced by F1 x F1 matings. The genetic background of C57BL/6J x 129 F2 mice varies among littermates because of gene segregation from the F1 hybrid parents. Although these F2 mice are only an approximate genetic match to the B6;129 background, they do contain only genes derived from either the C57BL/6 or 129 genetic backgrounds. We currently distribute two C57BL/6J x 129 F2 hybrids, differing primarily in their 129 substrain progenitor:
- B6129PF2/J (Stock Number 100903)
Parental strains: C57BL/6J-Aw–J and 129P3/J (formerly 129/J).
Suggested control for strains designated B6;129P. - B6129SF2/J (Stock Number 101045)
Parent strains: C57BL/6J and 129S1/SvImJ (formerly 129S3/SvImJ).
Suggested control for strains designated B6;129S.
The controls noted in the strain details for mice carrying targeted mutations are selected to most appropriately match the 129 strain used to derive the ES cell lines (Simpson et al. 1997).
Note: The B6129F1 hybrids are usually less appropriate controls than are the F2 hybrids because the parental alleles of F1 mice are not segregating as opposed to those on a mixed B6;129 background.
Controls for Mice with Spontaneous Mutations
For mice with spontaneous mutations on either a mixed or segregating genetic background, controls from the colony are best. In many cases, control mice may be untyped (i.e., the genotype is not determined by either DNA analysis or progeny testing). Untyped control mice do not display the mutant phenotype and are either heterozygous (+/–) or wild-type (+/+) for the recessive mutant gene. Because genotypes of untyped mice are not known, they are designated (+/?). Because many strains of mice with spontaneous mutations are maintained homozygously, controls are not available from the colony. If the homozygous colony is maintained on an inbred background, the inbred strain should be used as the control. F2 hybrids must be used as controls for homozygous colonies maintained on a mixed C57BL/6 x 129 background (designated B6;129). In some cases, a strain with a spontaneous mutation is maintained homozygously on a unique genetic background strain for which there is no wild-type control. For these strains, it may be necessary to choose a related strain as a control.
Controls for Congenic Strains
Because the introduced genes on most JAX® Mice congenic strains are maintained homozygously, standard inbred strains must be used as controls. The degree of similarity between the congenic and the background inbred strain depends on the number of backcrosses performed to produce the congenic. Statistically, after 10 generations of backcrossing (N10), the congenic and the inbred background stain are 99.9 % similar at all unlinked loci. However, the region surrounding the introduced gene contains approximately 10 centimorgans of residual donor DNA. The background strain is also considered the appropriate control for incipient congenic strains (N5 to N9). However, histocompatibility between the background strain and the incipient congenic cannot be assumed.
References
Simpson EM, Linder CC, Sargent EE, Davisson MT, Mobraaten LE, Sharp JJ. 1997. Genetic variation among 129 substrains and its importance for targeted mutagenesis in mice. Nat Genet 16:19-27.
