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]