A lot of work has been done in the breeding of mice for laboratory purposes (inbred lines are desirable so that there's as little variation as possible in scientific experiments), for this reason there are some standard definitions that apply to breeding. I don't know how these apply across different species. This is what I found in _Mouse Genetics_, page 41, by Lee Silver: "The major hurtle that must be overcome in the development of new inbred strains from wild populations is inbreeding depression, which occurs most strongly between the F2 and F8 generations. The cause of this depression is the load of deleterious recessive alleles that are present in the genomes of wild mice as well as all other animal species [an allele is one copy of a gene or locus, recessive genes are expressed if there are two recessive alleles present, but are not expressed if there is a dominant allele present]. These deleterious alleles are constantly generated at a low rate by spontaneous mutation but their number is usually held in check by the force of negative selection acting upon homozygotes. With constant replenishment and constant elimination, the load of deleterious alleles present in any individual mammal reaches an equilibrium level of approximately ten. Different unrelated individuals are unlikely to carry the same mutations, and as a consequence, the effects of these mutations are almost never observed in large randomly mating populations. Thus it is not surprising that during the early stages of mouse inbreeding, many of the animals will be sickly or infertile. At the F2 to F8 generations the proportion of sterile mice is often so great that the earliest mouse geneticists thought that inbreeding was a theoretical impossibility...." So I know at least in mice the terms inbreeding and inbred have a little bit different of a meaning. The definiton of inbred to a mouse geneticist means- animals that result from the process of at least 20 sequential generations of brother-sister matings. Inbreeding- is the process used to get there. As mentioned above the process of inbreeding can lead to some severely bad crosses. And like the anon. poster stated culling, or at least not breeding the results of the bad crosses, is necessary for inbreeding to be succesful. I can see how inbreeding can be confusing for a person not used to thinking in these terms. Basically, inbreeding for the short term is bad, but inbreeding for long term (many generations) can bring out desired phenotypes of animals, but it is also pretty well accepted that inbred lines of mice are almost never as hardy as a wild strain, and often in mouse genetics hybrid strains are used in experiments because outcrossing to a different strain usually leads to a much healthier and productive mouse. It should also be obvious that maintainace of the inbred line is just as important as generating the line to begin with. Mutations build up over generations, and in an inbred line there are no fresh genes coming in from the wild. This may have been more confusing than anything else. Leigh [Posted in FML issue 2618]