be represented not by the allele(s) of interest but by the most proximal and distal markers identified in the donor interval, such as in the strain B6.129S1‐(rs13480546‐rs13480629)/Kjn [25]. If two alleles of interest from separate genetic origins are combined into one congenic strain or if the precise origins of the donor sequence are not known, then there is no single donor strain that can be put after the period so that complex genomic donor source is represented as “Cg.” The Tg(Ins2‐rtTA)2Doi transgenic mice were generated by microinjection into (C57BL/6J x SJL/J)F1 hybrid mice then backcrossed onto a NOD host to generate the congenic named NOD.Cg‐Tg(Ins2‐rtTA)2Doi/Doi [26], in which the Cg is used because it is unknown whether the congenic interval derives from C57BL/6J or SJL/J. If the source of the desired donor sequence does not come from a strain that is a pure inbred, but rather contains contributions from a source different from the host background or the donor congenic interval, then that strain information can be represented in parentheses after the donor strain abbreviation. Thus, if it were known that Tg(Ins2‐rtTA)2Doi inserted into C57BL/6J‐derived sequence in the original F1 hybrid, and not into SJL/J‐derived sequence, then the strain name of the congenic would be NOD.B6(SJL)‐Tg(Ins2‐rtTA)2Doi/Doi. This allows the reader to see at a glance that there is a possibility that some small amounts of SJL/J‐derived sequence exist in the strain. In the example of NOD.129S4(B6)‐Art2atm1Fkn Art2btm1Fkn/Lt it is immediately clear that both targeted alleles were generated in 129S4, at least one, if not both, were bred at least once to C57BL/6J, and then both were backcrossed onto NOD, either together or separately then bred together [27]. If there are more than one additional sources of trace contribution to the genetic background, aside from the donor, then Cg is entered into the parentheses to represent this. For example, NOD.129P2(Cg)‐Il10tm1Cgn Il4tm1Cgn/Dvs had C57BL/6J and C57BL/10J in part of the genetic lineage so the potential contributions from those backgrounds are captured in the (Cg) [28]. Another example of a congenic strain in which the donor origin is represented by “Cg” is B6.Cg‐KitlSl Krt71Ca/J, a C57BL/6J host congenic for both KitlSl, which arose in C3H/He, and Krt71Ca, which arose in a Swiss stock [29].
Table 3.9 Congenic strains.
| AdvantagesGenetic and phenotypic uniformityReduce experimental variabilityCan transfer most mutations onto a different genetic backgroundAllows examination of modifier genesCan maintain mutation or transgene homozygously and use inbred as control |
| DisadvantagesGeneration time (two to three years to N10 or longer if ovarian graft to immunodeficient mouse used; can reduce time with speed congenics)Phenotype may change on different genetic backgroundLinked genes may confuse experimental findings |
As illustrated in the above examples, more than one gene can be selected for in the congenic breeding process, especially if molecular markers are available so that each generation can be screened. The entire process is speeded up if markers for the host strain are identified on all chromosomes and selected for during the inbreeding process, to create what are termed speed congenics.
The advantage of creating congenic strains is that the gene of interest is now on the inbred strain the investigator wants to work with. The disadvantage is that there is always some remaining DNA from the donor strain around the gene that has been moved and potentially elsewhere in the genome. The more the strain is backcrossed to the new strain, the smaller this interval is, but some donor DNA still remains, and may be in small segments of forced heterozygosity not linked to the congenic interval. Endonuclease‐mediated technology can be used to replicate the same allele sequence in different inbred strains, which eliminates the problem of flanking donor sequence in the congenic interval and saves years of breeding.
There are also a group of strains generated by the outcross of a coisogenic mutant subline followed by backcrossing to the original inbred strain in order to return to something close the original genetic background. This can occur by error or by necessity, such as when the chimeric founder of a targeted mutation created in an ES cell derived from 129S6/SvEvTac is bred to C57BL/6NTac to select mice with an agouti coat color, indicative of transmission from 129S6. If it is desirable to assess the phenotype of the allele on a 129S6 background, then repeated backcrossing to that original background can produce something similar to a congenic in that it is predominantly 129S6 but may carry traces of C57BL/6NTac sequence even though the interval surrounding the mutation is 129S6. Such a strain would be named 129S6(B6NTac)‐Genetm1Lab/Lab with the abbreviation of the predominant strain followed by the abbreviation of the potentially contaminating strain in parentheses. This nomenclature says at a glance that this is not a congenic strain (no period), the allele did not originate in C57BL/6NTac, but rather in 129S6‐derived sequence, but that some contribution from C57BL/6NTac may be present in the genome. This nomenclature is only used after five backcross generations. Prior to that the correct strain nomenclature would be 129S6;B6NTac‐Genetm1Lab/Lab.
Consomic Mice
Consomic strains have one intact chromosome from the donor strain transferred to a host background through repeated backcrossing, similar to congenic mice, but with careful screening at each generation for a fully intact specific donor chromosome. The strain nomenclature for consomic strains places the donor strain as a superscript to the transferred chromosome. The strain C57BL/6J‐Chr 1PWD/Ph/Fore had Chromosome 1 from the inbred strain PWD/Ph bred onto a C57BL/6J host background by Dr. Jiri Forejt, whose laboratory code is Fore. A complete set of consomic strains, in which each chromosome from the donor strain is separately transferred to the same host background in a panel of separate strains, provides a unique tool for mapping mutations and modifiers.
These types of mice can be quite valuable when multiple congenic strains are created, each carrying the same single gene mutation. If each strain develops different lesions or different levels of severity of lesions, then one can integrate this type of strain into the analysis. For example, a spontaneous hypomorphic allele of laminin gamma 2 (Lamc2jeb) resulted a model for non‐Herlitz Junctional Epidermolysis Bullosa, a blistering skin disease. Five congenic strains homozygous for this mutation revealed very different onset and severity of blistering disease and other lesions [30]. By crossing the congenic strains, several quantitative trait loci (QTLs) were identified, one major modifier gene on chromosome 19. By creating B6 and C57BL/6J‐Chr 19PWD/Ph/ForeJ congenic strains that were both homozygous for Lamc2jeb (on Chromosome 1), these mice could be crossed and each time rearrangements only occurred on Chromosome 19, thereby shortening the interval when selecting for disease severity. In so doing, it was possible to reduce the genetic interval to 1 megabase thereby identifying the binding region of Col17a1 as the major modifier gene [31].
Conplastic Mice
Conplastic strains have been created by backcrossing the nuclear genome from one inbred strain into the cytoplasm of another in which the mitochondrial parent is always the female parent during the backcrossing program. The strain designation is the nuclear genome strain‐mtcytoplasmic genome strain. For example, C57BL/6J‐mtA/J/Na is a strain with the nuclear genome derived from C57BL/6J and the cytoplasmic genome from A/J, which was created by crossing male C57BL/6J mice with A/J females [32]. Their female progeny were repeatedly backcrossed to male C57BL/6J. As with congenic strains, a minimum of 10 backcross generations are required.
Outbred Mice
Wild mouse populations are generally not inbred, and some populations have great genetic diversity between individuals. Exceptions occur when there is severe geographical isolation in which gene diversity is mixed but less so than in large free‐ranging populations. The result of outbreeding is various degrees of genetic and therefore phenotypic variability between individuals (Table 3.10). Since human populations, in general, are outbred, properly maintained outbred mouse stocks can, in