Development of tools for the analysis of genetic variation dates back to the tumor transplantation studies conducted by Jensen and Loeb around 1900.10 These early observations, especially the hunt for histocompatibility genes during the 1940s, revealed a surprising amount of genetic variation in common mouse stocks. Discovery of this variation led to the large collection of inbred strains, the basis of all mouse genetics. In the introductory chapter to his classic book on The Origins of Inbred Mice, Herbert Morse quotes Hans Gruneberg, another mouse geneticist of repute, who said ''The introduction of inbred strains into biology is probably comparable in importance with that of the analytical balance in chemistry.'' The concept of congenic strains was conceived by George Snell at the Jackson Laboratory in the 1940s as a tool to dissect histocompatability genes, and recombinant inbred strains were developed in the 1970s by Don Bailey and Ben Taylor, also at the Jackson laboratory, as a further tool for unraveling complex traits in mice.
Recombinant inbred (RI) strains are a set of strains that has been derived from the cross of two unrelated but highly inbred progenitor strains and maintained independently under a regimen of strict inbreeding since the F2 generation. After 20 generations of systematic brother-sister inbreeding, each new inbred strain that is established contains a unique mixture of the genes of the two parental strains. Once inbred, a given RI strain represents a stable population in which all of the alleles are homozygous. Unlinked genes are randomly distributed in the F2 generation, while linked genes will tend to become fixed in the same combinations as they were in the parental strains. A set of RI strains derived in this way from the same pair of parental strains as shown in Figure 9.1 provides the tools for genetic analysis of traits.
On reflection, it will be seen that the common inbred strains represent collectively a large set of RI strains.
Individual strains within a set are customarily named by an abbreviation for the maternal parent strains followed by a capital X followed by the abbreviation for the paternal strain. For instance, in the set derived from maternal A strain mice and paternal C57BL/6 mice, the abbreviations are A and B, and the individual RI strains are designated AXB-1, AXB-2, AXB-3, etc., while those created from maternal B6 and paternal A mice are designated BXA-1, BXA-2, BXA-3, etc.
A long list of studies demonstrates the usefulness of RI strains in single-gene analysis, gene linkage, and gene mapping. An individual RI strain can be typed for specific differences that distinguish the parental strains, and all loci in a given set have a particular pattern called the strain distribution pattern (SDP). Typing of individual strains indicates which of the alleles from the parental strains is fixed in a given strain, and it need be done only once for a particular locus. In a typical analysis of a new genetic variant, the RI set then needs to be typed only for the new locus. Linked loci will often have SDPs that are similar or identical and they then become candidates for conventional methods of linkage analysis. If no linkage is detected, it may be necessary to await the identification of additional differential markers that may reveal linkage.
Once the SDP of an RI set has been determined, linkage of new loci to known loci can be sought by comparisons of their SDPs. Collation of SDPs for many loci for established RI sets was initiated long ago. The fact that the data on SDPs obtained are cumulative affords RI sets an enormous advantage over conventional crosses for mapping and linkage analysis. There is one major drawback of RI strain analysis, namely that only loci that differ in the parental strains can be analyzed. However, the rapidity with which strain differences are being reported at the DNA level by the application of molecular biological and related techniques makes it possible to recognize differences that heretofore were inaccessible.
The concept of congenic strains was conceived and developed in the 1940s and 1950s by George Snell as an experimental method by which he sought to reveal the individuality of genes.11 Congenic strains are genetically identical except for a short chromosomal segment, and hence these strains enable the effects of particular genes to be studied as free as possible from the effects of background genes.
A congenic strain is created by crosses between an inbred partner strain that donates the genetic background and another inbred strain that donates the chromosomal segment followed by a succession of backcrosses. Construction of a congenic strain is illustrated for the A.B6-Natr rapid acetylator congenic strain (abbreviated A.B6) in Figure 9.2. In accord with the protocol shown there, A and B6 inbred mice are crossed and the heterozygous F1 progeny are crossed back to A (background strain) mice. This scheme is repeated for 12 generations; the heterozygous (rs) progeny are selected from each generation for the next backcross, and the homozygous (ss) progeny are discarded. Theoretically, one-half of the B6 genes are lost at each backcross, and at the 12th generation '/212, or less than
B6 STRAIN (M) A STRAIN (F) DONOR BACKGROUND
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