The remarkable advances in the science of genetics over the past few decades are having a strong impact on laboratory animal science. These advances are due to the development of a range of molecular techniques making it possible to map, clone, and sequence many genes, but progress has been facilitated by the classical period of mouse genetics which started soon after the rediscovery of Mendel's work in 1900 and continued until about 1980. This laid a firm foundation on which the new molecular methods could be based. A large proportion of "Laboratory Animal Genetics" is in fact "Mouse Genetics."1 This should not imply that the genetics of other species are unimportant, but simply that for many technical reasons, including small body weight, high reproductive performance, small space requirements, and the availability of a wide range of strains and mutants, the mouse has been used more extensively than any other mammalian species.
Following the rediscovery of Mendel's paper in 1900 setting out the laws of inheritance of discrete characters in garden peas, the validity of these laws was soon confirmed in the mouse by Cuenot2 and others using coat color in pet mice as the unit characters. Since that time, visible mouse mutants have often been preserved, even if the mutation appeared to be bizarre and have no obvious biomedical significance.
The development of inbred strains of mice by C.C. Little and rats by Wilhelmina Dunning both in 1909 and guinea pigs by G.M. Rommel, later taken over by Sewall Wright, starting in 1906, was a major advance, because for the first time pure-breeding lines became available.
Several of these strains, and ones developed by other investigators in this early period, are still available and are widely used in biomedical research. Inbreeding, usually due to many generations of brother and sister mating, has the effect of fixing the genotype within a strain, while maximizing the differences between strains. Some of these mouse strains were selected for a high incidence of various types of cancer, such as mammary tumors in C3H, and DBA, leukemia in strain AKR, and lung tumors in strain A. These strains were widely used in cancer research.
As early as 1903, it was found that tumors could often be transplanted within the strain in which they originated, without being rejected, but they were usually rejected when transplanted into a different strain. These early studies had been done using Japanese waltzing mice, which had apparently become inbred accidentally by fancy mouse breeders. Subsequent studies by Little and Tyzzer3 showed that tumor rejection was dependent on a number of genes with a dominant mode of inheritance.
George Snell, at the Jackson Laboratory in Maine, continued these studies and identified some of the gene loci responsible for this rejection by backcrossing the ability to resist tumor grafts from one strain into a strain where the grafts would normally be accepted (called the inbred partner). His so-called "congenic-resistant" stains were usually found to differ from the inbred partner at a single genetic locus. In most cases, this was what is now known as the major histocompatibility complex (MHC), also designated the H2 locus in mice. Not only did Snell identify this important locus, but his work also promoted the use of backcrossing to an inbred strain as a method of fixing the genotype in order to provide stable material for further study. Snell and his generation of transplantation immunologists developed several hundred of these congenic-resistant strains, which are still widely used in research involving the immune system. Congenic strains developed by backcrossing mutants and, more recently, transgenes to an inbred genetic background are now widely used.
Another significant advance was the development of sets of "Recombinant Inbred (RI) Strains" by Donald Bailey in 1971.4 He crossed two standard inbred strains and then brother x sister mated the offspring so as to produce a whole set of new inbred strains in which the genes from the parental strains had recombined. His first set consisted of seven strains derived from a cross between inbred strains BALB/c and C57BL/6By. Larger sets of strains were later developed by B.A. Taylor,5 and were used extensively first to determine whether a given phenotypic (i.e., observed) difference between strains could be attributed to a single genetic locus, and if so, whether it was linked to any known genetic markers. These sets of RI strains have been used to identify and map gene loci, which were polymorphic between the two parental inbred strains. Their use is discussed in more detail below.
RI strains are quite good for resolving the genetics of characters controlled by one or two loci, but they have some limitations for studying many characters with a polygenic mode of inheritance (i.e., where the phenotype depends on the joint action of several gene loci as well as nongenetic factors). This led Demant6 to develop sets of "recombinant congenic" strains specifically for this task. These sets of about 20 strains typically differ from an inbred partner by about 12.5% of those loci originally polymorphic between a donor and inbred partner strain. Though not yet widely used, they illustrate how specific strains can be developed as useful tools in biomedical research.
An important feature of this classical period was the development of flexible and adaptable nomenclature rules for inbred strains, genetic loci, alleles, mutants, and chromosomes. These are administered by international nomenclature committees for mice and rats, which try to ensure that the same strain, mutant, etc., is not named differently by different investigators, or conversely, that different things do not end up with the same name. With rats, three competing nomenclature systems for the rat MHC managed to become established, and it took considerable effort for the rat nomenclature committed to reconcile them into a new system.
The maintenance of ever-increasing numbers of mouse and rat strains is expensive in terms of space and scientific resources. This has been alleviated to some extent by the development, toward the end of this classical period, of methods of freezing mouse embryos.7 These can be maintained in liquid nitrogen for many years, saving considerably on maintenance costs and space. The development of associated methods of handling preimplantation embryos has also had important consequences for the development of transgenic strains.
The development of molecular techniques from the 1980s, largely driven by the Human Genome Project, has had a number of important consequences. The cloning and sequencing DNA taken together with the development of methods for handling early embryos led to the development of methods for producing transgenic strains following the injection of foreign DNA into the pronucleus of early embryos.8 Later, embryonic stem cells lines were developed from cultured early embryos.9 These could be maintained and manipulated as cell cultures, allowing gene targeting by homologous recombination in order to develop "knockout" mice, in which a specific gene was inactivated.10 This has proved to be a powerful tool in finding out the functions of many genes, but it has also resulted in a remarkable proliferation of new strains.
Microsatellites are short repetitive DNA sequences with unique flanking sequences. These are widely distributed throughout the mouse and rat genomes and provide a large number of genetic markers, which have been used for genetic mapping and genetic quality control. These markers can also be used to map so-called "quantitative trait loci" (QTLs), which are loci controlling the inheritance of many complex characters, such as susceptibility to many toxic agents and diseases such as cancer, diabetes, and various aspects of behavior. Once these genes have been mapped to a general chromosomal location, they need to be identified. This usually requires extensive backcrossing programs using the methods used by Snell in developing his congenic-resistant strains. Unfortunately, such backcrossing takes two or three years, but by using an array of microsatellite markers, "speed-congenics" (see below) can be developed in about half the time, though at some cost in organization, testing, and reagents.
Keeping track of the vast amount of genetic information now being generated on gene sequences, polymorphic genetic markers, genetic maps, new strains, and phenotypes would have been impossible without the parallel development of informatics, which developed as a separate discipline from about the mid 1990s. The full impact of the World Wide Web has yet to be felt, though it is already substantially altering the way that science is done and communicated. Already, there are a large number of Web sites offering information and resources at locations throughout the world. Useful Web sites of potential value to mammalian geneticists include: Quantitative Genetics Resources (http://nitro.biosci.ari-zona.edu/zbook/book.html), World of Genetics Societies (http://www.faseb.org/genetics/), genetics-related Web sites (http://www.sidwell.edu/sidwell.resources/bio/VirtualLB/bioIweb.html), Genetics Education Center (http://www.kumc.edu/gec/), statistical genetics Web sites (http://www.rdg.ac.uk/~sns99kla/links.html), and animal behavior Web sites (http://www.societ-ies.ncl.ac.uk/asab/websites.html). A computer program, "MICE," which is used for automation of breeding records and is distributed free of charge to academic institutions, is also available through the Web (www.biomedcentralxom/1471-2156/2/4).
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