strain
Sibling breeding F1 hybrid mice for one generation produces an F2 hybrid population (Figure 3.4), and continued inbreeding after that eventually leads to the creation of recombinant inbred strains (see below). The F2 hybrid population also has hybrid vigor, but is distinct from the F1 hybrid population because meiosis has resulted in recombinations between the parental chromosomes such that there may be some homozygosity in a minority of the segregating alleles. This makes an F2 hybrid population a tremendous tool for mapping, especially for mapping recessive mutations. For example, if an interesting phenotype is observed in an inbred strain, such as an abnormal hair coat, hair interior defect (hid) in all AKR/J mice, these animals can be crossed with another inbred strain, such as BALB/cByJ, that has normal hair (Figure 3.5). As hid in this example is an autosomal recessive mutation, the obligate heterozygous F1 progeny of this cross will all have a normal hair coat. When these heterozygous progeny are intercrossed to create F2 hybrids, 25% of these mice are expected to have the abnormal hair phenotype. The genetic interval carrying the mutant sequence derives from AKR/J so by screening the DNA from the mutant and unaffected mice in the population with molecular markers distributed evenly across all chromosomes, a pattern emerges in which markers specific to AKR/J and not BALB/cByJ skew to increased homozygosity in the region of the genome specific to the mutation. Usually this works relatively quickly to identify the region in which the mutated gene is located. However, in this particular case, less than 15% of the F2 mice were affected (Table 3.7) due to modifier genes in BALB/cByJ. Similar crosses with other strains resulted in the expected 25% distribution. This illustrates two points, the deviation from Mendelian expectation due to modifiers and the value of doing mapping crosses with different inbred strains, which in this instance identified different genetic intervals that together reduced the interval and the number of candidate genes (Figure 3.6) [9, 10].
Figure 3.5 Setting up a mapping cross using F1 and F2 hybrids.
Backcrossing an F1 hybrid to one of the parents, instead of sibling intercrossing, is the initial process in the creation of a congenic, consomic, or conplastic strain, and such an N2 population is a useful tool for mapping dominant mutations. Some mutations or transgenes are adequately deleterious to overall health that they cannot be maintained on an inbred background and instead require hybrid vigor. Such mutants are maintained by breeding to an F1 hybrid at each generation or through outcross–intercross to an F1 hybrid. One complicated example of this is B6EiC3Sn a/A‐Ts(1716)65Dn/J [11]. This chromosomal aberration was induced in DBA/2J inbred mice, but is detrimental to health and causes male sterility so in order to successfully maintain a colony a female carrier is bred to a (C57BL/6JEiJ x C3H/HeSnJ)F1/J male at each generation. There is no punctuation in the genetic background of the strain name, B6EiC3Sn, and the presence of the DBA/2J contribution is omitted from the nomenclature for simplicity and must be found in sources outside of the strain nomenclature.
Table 3.7 Breeding data used to identify the gene responsible for the hair interior defect phenotype.
| AKR/J‐hid* homozygote crossed with | Total progeny | Normal mice | Affected mice | Affected (%) | X 2 | p‐Value |
|---|---|---|---|---|---|---|
| BALB/cByJ | 260 | 222 | 38 | 14.62 | 5.92 | 0.025–0.010 |
| C57BL/6J | 430 | 330 | 100 | 23.26 | 0.220 | 0.500–0.750 |
| CAST/EiJ | 339 | 255 | 84 | 24.78 | 0.003 | >0.900 |
| C3H/HeJ | 248 | 191 | 57 | 22.98 | 0.170 | 0.500–0.750 |
| FVB/NJ | 272 | 205 | 67 | 24.63 | 0.006 | >0.900 |
| Total F2 mice | 1549 | 1203 | 346 |
* AKR/J‐hid was the original designation until the gene responsible was identified. Because the gene was identified with a different phenotype earlier, the allele is officially Soat1ald not Soat1hid.
Figure 3.6 Crossing AKR/J mice with more than one inbred strain to map the mutant gene locus. By using the shortest interval between different crosses, the list of candidate genes could be reduced. Sequencing then only had to be done on a few genes to identify the mutant gene responsible for the hair interior defect phenotype.
Sources: Based on Giehl KA et al. [9].
Some mutations are maintained on a background in which the genetic contributions of two inbred strains are segregating. The retarded hair growth (in ornithine aminotransferase, Oatrhg) mutation arose spontaneously in AKR/J and was later outcrossed to C57BL/6JEi and subsequently maintained by sibling intercrossing [12, 13]. Thus, the genetic background consists of contributions from both AKR/J and C57BL/6JEi segregating at each generation. A semicolon between the strain abbreviations is used to represent this situation, B6Ei;AKR‐Oatrhg/J.
