Breathing a little better today w the hepa filter air cleaners going.
Before someone who is not familiar w Neural Crest Genetic Variations
jumps in I just want to point out that while deafness itself is not a
deformation, the physical cause of the deafness in such a situation is.
(The spell checker is turning "hepa" into so many other words today...)

Neural Crest Genetic Variations cause cells derived from the VERY EARLY
(even before organ formation) Neural Crest section of the embryo to be
altered. How much, and in what ways, varies among individuals. The
extremely early embryo has several crests and each in the precursor for
multiple types of cells, organs, structures in the body, so changing
those early cells introduces an assortment of possible alterations
but they vary among individuals depending on which cells are changed,
how many genes for such alterations are present, etc. That is called
Variable Expression because the genetic variation's expression varies.
Changes to the cells derived from pigment cells are common in these
particular individuals though, and those changes to cells derived from
pigment cell cause both the extraneous white patches and deafness in
the individuals. An increase in deafness when those extraneous white
patches are on the head is very similar to a presentation seen in
Dalmatian dogs who have more deafness in those whose heads, and
especially ears, are whiter.

Breeding for Neural Crest Variations is simply asking for health and
longevity problems.

Thirty years ago before there were so many fancies, and mostly just
standard markings or albinos in the U.S. the reports of a range of
illnesses was lower, the ages at which those onset was older, deafness
was very rare as were a number of other problems, people were horrified
by any ferret passing who was younger than very late in the sixth year,
and eight year old ferrets were not unusual.

[Posted in FML 8093]