Selective breeding for many centuries has resulted in the establishment of well-defined dog breeds that are based on features such as head type, overall size, coat, color, and body structure. The development of different breeds has been possible because of a phenomenon called genetic mutation. Mutations occur spontaneously, often for reasons unknown. A mutation changes encoded information in a gene, which results in the addition or deletion of an enzyme or protein that may perform an important cellular function. When a mutation arises in an egg or a sperm cell, it affects all the cells in the embryo and, if the animal survives, the mutation is passed on to future generations.
The majority of mutations are detrimental in that they do something that interferes with the animal’s ability to adapt to his environment. In nature, such mutations usually do not become part of the gene pool, because the individuals possessing them do not survive to pass on their genes.
As wild dogs became dependent on humans for survival, matings between the most highly adapted and competitive animals gave way to a system in which breeding partners were selected by their owners for specific reasons. When mutations occurred, they often changed the appearance of the dog, but may also have given the dog a better sense of smell or the ability to burrow in pursuit of rodents. By selectively breeding such dogs to others of similar type or attribute, over a long period of time humans were able to create hundreds of breeds as diverse as Saint Bernards, Chihuahuas, Pugs, and Salukis. Despite differences, these are all still dogs and are capable (at least in theory) of interbreeding.
Unfortunately, selective breeding has also resulted in the appearance of a number of genetic diseases, involving most of the organs and systems dis- cussed throughout this book. (All of these mutations may also be seen in mixed-breed dogs, as well.) Sadly, these diseases are accepted and tolerated as part of some breed packages, even though they weaken the species and have ethical as well as financial consequences to dog breeders and owners. Inherited diseases can, however, be controlled (and in some cases even eliminated) through informed breeding practices.