Purebred dogs and genetic diseases

Why are genetic diseases so frequent in purebred dogs?

In most modern canine breeds the level of inbreeding is high. This is due to a low number of animals generally used to create a breed. It is often followed by the intensive use of a few popular animals to produce puppies. So if a harmful mutation was present in one of the founder animals and moreover if this mutation is inherited by one popular sire it will be transmitted to a lot of dogs! Consequently the frequence of the mutation is high in that breed and the chance that two dogs carrying the mutation are bred together is rising. When it is the case it leads to a few affected puppies in the nest.

What is the proportion of affected puppies if the mutation is dominant?

A dominant mutation means the disease appears with one copy of DNA affected. So at least one of the parent must also be affected. But for some disease (as eye disease) the  symptoms may occur later, once having already bred the dog (at mid-age, 5 to 6 years old). Half of its reproductive cells carry the mutation. If this animal is bred with a healthy partner, half of the puppies will be affected, the other half will be completely healthy. If we deal with a dominant mutation, affected animals must be discarded from breeding schemes.

What is the proportion of affected puppies if the mutation is recessive?

In a  recessive disease the two DNA copies must carry the same mutation to see the appearance of a disease. Usually the parents are healthy but they carry one copy of the mutation (they are called CARRIERS and they transmit the defect to half of their reproductive cells. If two carriers are bred together the chance that an affected sperm encounters an affected egg cell is 1/2 * 1/2 so 1/4. One fourth of the puppies in average will be affected. Carriers can be kept for breeding only if they are bred with healthy dogs and the status of the resulting puppies must be checked as half of them will be carriers!

Which are the breeds affected?

Nearly all breeds have several genetic diseases segregating at various level. A list of specific disease for each breed can be found at the two following links:

Canine Inherited Disorders Database
Inherited Disease in dogs – Cambridge University