Since
CNM is inherited as a simple recessive trait in Labradors, the disease
allele must be inherited from both parents in order to cause CNM (Labrador
Muscular Myopathy) in a pup. Whenever two carrier dogs are bred, there
is a possibility of producing affected pups and more carriers.
On average, a litter from two carriers would produce 25% affected pups,
50% carriers, and 25% non-carriers. If a carrier was bred to a non-carrier,
then no pups would be affected, but 50% of the pups would be carriers.
These percentages are averages from large samples. In a normal sized
litter there may be a larger or smaller percentage in each category.
Since affected dogs look so different physically and lack the ability
to deal with cold and harsh environments compared to normal Labradors,
they would never be bred, even though they are capable of reproducing.
It
is expected that the CNM recessive will be only the first of many recessive
markers available to breeders of Labradors. There will be an enormous
need for public education regarding breeding practices due to the rapid
changes in genetic information that will soon be available.
One option for breeders would be to screen litters for carriers and
non-carriers before placement. Carriers could be sold to those buyers
who were not interested in breeding. The carriers would have all the
strengths and positive characteristics potential in the pedigrees. Non-carriers
could be bred without concern, at least for the CNM recessive.
It is widely believed that it would be a disadvantage to remove all
carriers of all potential recessives from breeding programs. Narrowing
the gene pool to remove all possible carriers of the many possible recessives
could result in a gene pool that is significantly narrow. Narrow gene
pools often allow new recessives, not prevalent or even recognized at
this time, to emerge. In addition, many of the highly desirable characteristics
that are sought and enjoyed in Labradors may be lost.
