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a Power analysis: number needed in a two-sample t-test to detect a 10% change in the mean (two-sided) with a 5% significance level and a power of 90%.

b Number needed to detect a 4 min change in sleeping time with a 5% significance level and a power of 90%.

may apparently alter strain characteristics, but this may be acting through associated microflora rather than through the genetics of the inbred strain. This long-term stability has important consequences. It means that genetic profiles will remain unaltered for long periods and phenotypic profiles of strain characteristics such as life span, types of spontaneous disease, immune functions, susceptibility to microorganisms, and biochemical and physiological characteristics can be built up by many research workers over a long period of time, provided the environment is also constant. However, some aspects of the environment, such as the associated microorganisms, diet, bedding, and social factors can alter strain characteristics, sometimes to quite a marked extent.

5. Individuality — Each strain is represented by a unique combination of alleles that confer a particular set of phenotypic characteristics. Sometimes these characteristics are useful in research, while at other times, they may preclude the use of a particular strain for a given research project. Many strains of mice were developed for use in cancer research and have high levels of a particular type of tumor. For example, strain AKR gets leukemia, SJL gets reticulum cell sarcoma, and C3H gets mammary tumors provided it carries the mammary tumor virus. However, other strains such as C57BL/6 have relatively low levels of cancer and tend to be relatively resistant to carcinogens. Differences in virtually any phenotypic characteristics are likely to be found among a group of independent strains. Many aspects of behavior such as open-field activity, wheel running, aggression, and learning ability differ between strains. Similarly, response to bacteria and viruses often varies dramatically between different strains. These strain differences have enormous importance, as strains can be chosen that have characteristics which make them useful for any given type of research. In extreme cases, whole areas of research have been opened up, based on the characteristics of a single mouse strain. For example, monoclonal antibody technology is all based on the plasmacytoma tumors induced in BALB/c mice by intraperitoneal injection of mineral oil, and the technology for gene targeting to produce "knockout" mice by homologous recombination, in which a single defined gene is inactivated, is based on the establishment of embryonic stem cell lines from strain 129 mice.

6. International distribution — The isogenic property of inbred strains means that daughter colonies can be set up using only a single breeding pair, and because of the long-term stability of inbred strains, these will remain similar to the parent colony for many generations. This means that work can be repeated using similar animals throughout the world. In contrast, although outbred stocks such as Wistar rats are available all over the world, genetic drift and directional selection mean that each colony is, to some extent, genetically different.

7. Identifiability — The isogenic property makes it possible to build up a genetic profile of each strain. This can then be used for genetic quality control purposes. Thus, if some white rats are supposed to be strain F344, this can be tested from a small sample of DNA using one of the methods discussed below (see genetic quality control). However, if the rats are supposed to be outbred Wistars, there is no known method of genetic quality control that will distinguish Wistars from Sprague-Dawleys.

8. Sensitivity — As a broad generalization, inbred strains tend to be more sensitive to environmental influences than outbred stocks or F1 hybrids (see below). This is a disadvantage in that it means that extra care is needed to ensure that they have the optimum environment. If not, they will tend to be more variable. However, it is an advantage in that they will also tend to be more sensitive to experimental treatments than other classes of stock.

Nomenclature of Inbred Strains

The rules governing genetic nomenclature for mice and rats are formulated and maintained by international committees made up of research workers with an interest in the genetics of these two species. The rules are subject to constant revision as knowledge of genetics increases and as different types of animals become available. For example, transgenic strains were not envisioned at the time the rules were originally formulated. Full details of mouse genetic nomenclature are given on the Jackson Laboratory Web site (www.informatics.jax.org) and of rats and mice on the Rat Genome Database Web site (www.rgd.mcw. edu).

Briefly, inbred strains of mice and rats are designated by a code consisting of uppercase letters (e.g., SJL, LEW) or (less preferably) letters and numbers (e.g., C57BL, F344), except for strains that were already known by some other designation at the time that the rules were formulated. Short symbols are preferred, and duplicate designations must be avoided. Strains do not have names, only designations. Many people using rats seem to want to name them, sometimes with names that are quite inaccurate. For example, the DA strain of rats is sometimes called the "Dark Agouti" strain, though it is not dark, and the "D" in the designation actually stands for the D blood group (now an obsolete nomenclature), not for the word dark. A name is confusing, as it does not conveniently fit with the rest of the genetic nomenclature, particularly with the need to designate substrains. Fortunately, users of inbred mice seem to accept designation codes rather than names.

Where necessary, the level of inbreeding can be designated by an F followed by the number of generations of full sib mating, e.g., F87, or F?+50 in the case of a strain with unknown inbreeding followed by 50 known generations of sib mating.

Substrains can arise when branches of an inbred strain have been separated after 20 generations but before 40 generations of inbreeding, or when branches have been maintained separately for 100 or more generations from their common ancestor. In both cases, some divergence is expected. Substrains are also formed when genetic differences between branches have been found. A substrain is indicated by a slash and a number, a laboratory registration code, or a combination of the two. For example, FL/1Re, FL/2Re are two substrains of strain FL, both established in the laboratory with code Re (see below). Some exceptions, such as the "c" in BALB/c have been permitted for strains already known using a different designation.

Laboratory registration codes are administered by the Institute of Laboratory Animal Research (ILAR), and codes can be registered directly on their Web site (http://dels.nas.edu/ilar/). They are used, as above, to indicate substrain differences and also to remind people that genetically identical animals raised in different environments, such as found in different laboratories, may differ phenotypi-cally due to nongenetic causes.

Research Uses of Inbred Strains

Inbred strains are, or should be, the animal of choice for general research using mice or rats. They are the nearest thing to a pure reagent that is possible when using laboratory animals. As noted above, a strain represents a single genotype, which can be repeated by the thousands. It stays genetically constant for long periods, there is considerable background information on the genetic and phenotypic characteristics of the more common strains, the phenotype tends to be uniform, and genetic quality control is relatively easy using a wide range of DNA genetic markers. If the project requires a screen of animals with a wide range of potential genetic susceptibility to a toxic or pharmaceutical agent or microorganism, then small numbers of animals of several strains can be used. This is much more effective than using an outbred stock, which in fact, is often phenotypically quite uniform and does not necessarily have a wide range of susceptibility genotypes. There are compelling arguments for using this approach.11,1213,34

Table 9.5 Estimated "Top 10" Inbred Strains3 of Mice and Rats

Rank

Mouse Strain Rat Strain

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