This article is from the Genetic Diseases in Dogs FAQ, by Gary Mason.
"There are no more easy problems." Anonymous
The need for accurate definition of the mode of inheritance - The
underlying causes of genetic diseases can be very complex. Efforts are
underway to identify and isolate specific genes, and combinations of
genes, related to various diseases. But it will probably be a very
long time before most have been isolated. The research process is
costly and very time consuming.
Variation in the expression of the disease - Genetic diseases which
appear to be identical across breeds may in fact be caused by
different genetic conditions. For example, vWD is believed to
exemplify one mode of inheritance in Scottish Terriers, and another in
German Shorthaired Pointers.
The absence of accurate tests - Some genetic characteristics can be
determined by observation, but more frequently tests are necessary to
identify specific genetic diseases. There are currently two tests for
vWD, one more recent (and accurate) than the other. They test for the
same constituent in the blood, but use different testing techniques.
These tests are based upon measuring the quantity of a specific
chemical in the blood, and while the test itself is very good, the
results are subject to substantial variation based upon the collection
and handling of the test samples. And there can be major variations in
the amount of the chemical present in the animal due to its condition
at the time of sample collection. Other genetic diseases depend upon
other methods for their diagnosis. These include X-ray, physical
manipulation, and other techniques. Testing for recognized genetic
markers, or the genes themselves, will offer a virtually foolproof
method for diagnosis. When a definitive test is developed for any
disease, there should be no reason to ever produce a puppy adversely
affected by that disease.