What are genetic tests?

Corgi's health
I DNA test all our dogs on vWD, Flaffy and eyes DNA for PRA. Of course, I also DNA test my dogs on Degenerative Myelopathy (DM). I do this to know what is going on in my lines and to make best combinations healthwise. I don`t want to exclude carriers or affected animals from my breeding but my goal is not breed new affected ones. 
We are lucky to have some DNA tests in the breed, therefore we can use the carriers and even the affected dogs and within some generations we can breed clear dogs again.
As more DNA tests will be available in the future, there’s also a danger.
In my humble opinion breeding should not become a routine by checking only the defects or disease. I think breeding as an art and as a breeder I have clear picture or wish-list in my head, because of which I try to find great combination all the time. 

What is DNA testing?
Canine genetic testing is the testing of the health by DNA testing. 
DNA tests are designed for diseases that are passed on from generation to generation, from parents to offspring. This is called the autosomal recessive type of inheritance. 
Let's understand it in more detail.
For an autosomal recessive disease to occur, the mutant copy of the gene must be inherited from both parents. If one of the parents has a mutant copy of the gene, the autosomal recessive mutation does not manifest itself, but such a dog is a carrier of the disease.

This is easy to understand:
both copies of the parents' gene are positive +/+ (clear)
only copy of one parent's gene is negative +/- (carrier)
Both parent's genes are negative -/- (affected)

Why do we need to do this?
First, for a competent breeding - for the positive dynamics of the breed. Genetic testing makes it possible for breeders to select dogs over several generations, eliminating the undesirable gene, but preserving the exterior and working qualities, without reducing the genetic diversity.

Welsh Corgi Pembroke and Cardigan are tested for diseases:
Degenerative Myelopathy / Degenerative Radiculomyelopathy) DM 
Canine degenerative myelopathy (also known as chronic degenerative radiculomyelopathy) is a progressive disease of the spinal cord in older dogs. The disease has an insidious onset typically between 7 and 14 years of age. It begins with a loss of coordination (ataxia) in the hind limbs. The disease is chronic and progressive, and resulting in paralysis.
Clinical Signs
Degenerative myelopathy initially affects the back legs and causes muscle weakness and loss, and lack of coordination. These cause a staggering effect that may appear to be arthritis. The dog may drag one or both rear paws when it walks. 
Genotype: N / N [ Homozygous normal ]
The dog is noncarrier of the mutant gene.
It is very unlikely that the dog will show signs of the Degenerative Myelopathy
Genotype: N / DM Heterozygous ]
The dog carries one copy of the mutant gene and one copy of the normal gene.
It is very unlikely that the dog will show signs of the Degenerative Myelopathy
Genotype: DM DM (Exon 2) [ Homozygous mutant ]
 The dog carries two copies of the mutant gene and therefore it will pass the mutant gene to its entire offspring.
The dog may or may not show signs of the disease

Hair length (“Fluffy”)
For the majority of registered dog breeds, such as Pembroke Welsh Corgis, the breed standard allows only one hair length., the occasional appearance of long-haired dogs (also called “fluffies” in this breed) has been a problem for breeders. Long-haired coat length is inherited a an autosomal recessiv trait, therefore dogs that are carriers of the long hair mutation will appear to be normal (short hair) themselves but will likely pass on the long-hair mutation 50% of the time.
Long hair is also know as Fluffy is some breeds.
The DNA test allows to distinguish between 3 possible genotypes:
1. L/L Short Hair having 2 copies of the normal short-hair allele 'L'.
2. L/l Short Hair carrying the long hair mutation - carrier having 1 copy of the normal short-hair allele 'L' and 1 copy of the long-hair mutation 'l'.
3. l/l Long Hair having 2 copies of the long-hair mutation 'l'.

Progressive Retinal Atrophy (rcd3 PRA)
Progressive retinal atrophy (PRA) is a leading hereditary cause of blindness in pedigree dogs. So called night-blindness.
Genotype: N / N [ Homozygous normal ]
The dog is noncarrier of the mutant gene.
It is very unlikely that the dog will develop Progressive Retinal Atrophy (rcd3 PRA). The dog will never pass the mutation to its offspring, and therefore it can be bred to any other dog.
Genotype: N / PRA [ Heterozygous ]
The dog carries one copy of the mutant gene and one copy of the normal gene.
It is very unlikely that the dog will develop Progressive Retinal Atrophy (rcd3 PRA) but since it carries the mutant gene, it can pass it on to its offspring with the probability of 50%.
Carriers should only be bred to clear dogs.
Avoid breeding carrier to carrier because 25% of their offspring is expected to be affected (see table above)
Genotype: PRA / PRA [ Homozygous mutant ]

von Willebrand disease Type I (vWD I)
Von Willebrand disease (vWD) is probably the most common inherited bleeding disorder in dogs. It is caused by lack of von Willebrand factor which is a protein that plays a key role in the blood clotting process resulting in prolonged bleeding.
vWD Type I is transmitted as autosomal incomplete dominant trait. This means that a dog that is genetically clear (also called homozygous normal) will not develop the vWD disorder and will not pass it to its offspring. Carrier dogs which carries one copy of the abnormal gene and another normal copy (also known as heterozygous) will have bleeding tendency. These carrier dogs will pass the abnormal gene to their offspring with a probability of 50%, the trait is called incomplete dominance because carrier dogs may not develop the disorder at all but they will still pass the abnormal gene to their offspring. Because it is very uncommon for carriers to show symptoms of vWD this condition is treated as Recessive. Affected dogs (carry two copies of the abnormal gene) will develop the vWD disorder and will pass the abnormal gene to each of their offspring

Like all dogs, Corgis do have some health concerns to watch for. By knowing about health concerns specific to Welsh Corgis, we can tailor a preventive health plan to watch for and hopefully prevent some
Early detection is the key to a long and happy life, so be sure to schedule routine checkups.