>As soon as there is any incidence of deafness, the entire bloodline
>should be shut down..  then, eventually there will be NO recessive gene
>left for deafness..(or at least that particular genetic strain of
>deafness)
 
Whoops, you forgot the math involved.  Remember that as you decrease the
number of animals with a recessive then there is less and less chance of
combining two who have a good chance of having the recessive hidden in
them to allow you to spot it through expression, so over time it becomes
harder and harder to spot even though it is still present in the
population.  So, you can't breed it out but you can reduce it.
 
With the neural crest things, though, the problem is more the variable
expression, though the hiding that creates has the same sort of
"difficulty in finding" effect.
 
So, these WILL remain, but the level of them in the population can be
decreased by careful breeding, but it going to be a good deal harder than
increasing the percentage was, as per reasons in other posts from others
and from me.  In countries where ferrets with non-standard spotting,
blazes, or panda heads are rare do you very best to avoid getting into
this mess.  In the U.S.  we need to spend a lot of time educating people
who do not think before breeding and need to try to decrease the
percentage.
[Posted in FML issue 4670]