Once upon a time
Once upon a time dog breeds were created by selective breeding to be suitable for specific tasks, such as herding, guarding and hunting. Today it is more likely that they are bred for their looks and to conform to written breed standards drawn up by a kennel club or a breed association.
These breed standards often include requirements for physical characteristics that are frequently detrimental to the dog’s health. This has led to Dachshunds, for example, having backs that are too long in proportion to their legs, making them more susceptible to spinal problems and Bulldogs, Pekinese and Pugs with faces that are too flat making it difficult for them to even breathe properly. One report states that over 50% of Cavalier King Charles Spaniels, which is the most popular toy breed in the UK, suffer from syringomyelia, which causes chronic head and neck pain. Another report on the same breed suggests that 59% of them will develop a heart murmur by the time they are 4 years old. The problems suffered by this breed are hardly surprising when one considers that all the Cavalier King Charles Spaniels existing in the world today are believed to have descended from only SIX breed founder dogs!
The exaggerated pursuit of conformity to breed standards has led to excessive inbreeding (breeders prefer the term “line breeding”) and therefore many diseases are inherited within breeds. Larger dog breeds suffer from inherited hip and elbow problems and a large number of eye problems are inherited by both large and small breeds.
Some experts believe that some pedigree dog breeds are being inbred out of existence and, given their lack of genetic diversity, if they were wild animals they would be on the endangered species list. Others have said that many breeds are literally falling to bits.
In a report published by the Genetics Society of America in 2008, which studied 10 representative dog breeds in the UK, the authors state that they found:
……extremely inbred dogs in each breed except the greyhound and estimate an inbreeding effective population size between 40 and 80 for all but 2 breeds. For all but 3 breeds, >90% of unique genetic variants are lost over six generations, indicating a dramatic effect of breeding patterns on genetic diversity.
Source: Population Structure and Inbreeding From Pedigree Analysis of Purebred Dogs, Federico C. F. Calboli, Jeff Sampson, Neale Fretwell and David J. Balding.