I didn't see anyone explaining the different of inbreeding vs. line
breeding and I thought I would include the definition as it was explained
to me by several breeders and from my 10+ years experience in breeding
ferrets.
 
Inbreeding is the result of putting closely related ferrets together
such as Brother /Sister, Mother/Son, Father/Daughter, Aunt/Nephew,
Uncle/Neice.  This is not a common practice but there are some breeders
who do not keep records that put these ferrets together.  There are also
some breeders who do not know the distinction between breeding ferrets
and breeding rodents (of which as I understand it, some inbreeding is
acceptable and would improve a rodents line (I don't breed rodents, this
is just my understanding)).
 
Line Breeding is the result of putting ferrets on the 3rd and 4th
generation together.  This is commonly used by breeders who are looking
to develop a particular color,size, etc.  This is the result of putting
grandparent/grandchild, cousins (1st or 2nd generation), great aunt/great
nephew, great uncle/great neice, etc.
 
Acceptable practice is varied greatly among groups and individuals.
Again, like Jen said, depends on how someone was taught.  I do not
condone inbreeding or line breeding and I personally believe due to my
background and my knowledge of genetics there should NOT be any cross or
introduction prior to 5 generations.  It is not considered line breeding
beyond 5 generations.  At this generation, the lines are a wash.  Prior
to the 5th Generation, chances of a birth defect are higher and may not
present itself right away.  It may not affect the Child or Grand Child
right away, it may affect their children or children's children.
 
Now keep in mind that many of these breeders have found difficulty in
locating lines that are of similiar likeness to their current breeding
stock and so they have resorted to line breeding.  Again, I do not
condone it but I understand their reasoning.
 
I myself have imported lines to help avoid using any line breeding and
have not line breed my lines.  I have been breeding for over 10 years.
 
I am not here to bash anyone and certainly there are reasons for why
someone might use these practices and again it is considered acceptable
in many groups or individuals.  This is an arguement that will probably
go on forever without an end in sight.  Just as others will say, everyone
has their own technique.  It's up to the person purchasing as to whether
they feel it is acceptable by their means or not.
 
Also, this maybe confusing but here's how my genetics professor broke
down the chances of birth defects to me:
 
First Generation  - offspring
Second Generation - Parents, Aunt/Uncle
Third Generation  - Grand Parents, Great Aunt/Uncle, 1st Cousins
Fourth Generation - 2nd Cousins, Great Grand Parents, Great Great
                    Aunt/Uncle
Firth Generation -  3rd Cousins, Great Great Grand Parents, Great 3X
                    Aunt/Uncle
 
Okay starting with:
 
1st Gen. - chances are 75% of birth defect (allow 25% error)
2nd Gen. - chances are 37.5%
3rd Gen. - chances are 18.75%
4th Gen. - chances are  9.375%
5th Gen. - chances are  4.6875%
 
Now this does not mean that each generation will have a birth defect
or that any at all may arrise.  These the calculations based on what
mathematics should say.  As we all know, this doesn't mean it is a
definite.  Genetics can be unpredictable and I am not a professor.
 
I do know from my genetic professor that the chances of it skipping a
generation and affecting further offspring is in some cases much higher.
 
Again, this is my opinion and knowledge of it.
[Posted in FML issue 4959]