Outcrossing, Linebreeding, and Inbreeding in Purebred Dogs
Part of the reason why designer dogs are becoming an ever more popular trend is the common perception that purebred dogs are inbred and unhealthy. In some cases, this is absolutely true. Plenty of breeders out there are doing a disservice to their dogs by producing sickly or even deformed puppies in search of money or an extreme type. However, it's certainly not true of all breeders, and the genetics of dogs is far more complicated than 'inbreeding = BAD!' In fact, almost every established breeder pairs dogs that have shared family. Below are the three major types of crosses, and why they are important to produce sound, predictable dogs.
Outcrossing is the mating of two dogs that are not closely related. In many ways, outcrossing two purebred dogs is similar to mixing two different breeds- you can't be sure what you'll get. Dogs with unrelated genomes can produce puppies that resemble one parent, both, or even neither. This may not seem like a big difference between two dogs of the same breed, but in the highly competitive world of dog shows, an inch of muzzle or a fractional difference of proportion can mean the difference between a championship or obscurity. Reputable breeders work to create a legacy- a line of dogs that better the breed as a whole and present 'typical' puppies. Because of this, outcrossing is generally only used when a breeder needs to bring in a strong trait not found in his or her own pedigrees, or to start a brand new line.
Linebreeding is the mating of related dogs, and is the most common type of pairing. By breeding within a family of dogs, breeders can select for desired traits and control the negative. Linebreeding is a slower process than inbreeding but is also less likely to produce deformed pups. At the same time, litters born will not have a great variation between puppies, allowing a breeder to slowly sculpt his or her ideal 'type' for the breed.
Technically, linebreeding is inbreeding, but not to a great degree. Common pairings would be grandparent/grandchild, cousins, or even half-siblings.
Inbreeding is the mating of two dogs that are closely related. This includes parent/offspring and sibling/sibling pairings. Inbreeding should only be done by a breeder with an in-depth knowledge of the pedigrees of both dogs.
Inbreeding is used to pin down certain desired characteristics in a line or breed, especially a particular gene. If two parents carry heterozygous genes (a dominant and recessive pair,) then there is a good chance some of the offspring will have homozygous recessive genes, and the breeder will then have dogs with the trait they want. There is, of course, also the chance that negative traits will pop up, and that is the risk of inbreeding. This means inbreedings must be carefully planned and done in moderation.
When planned carefully, inbreeding and linebreeding can be used to produce healthier dogs than an outcross. By putting a laser focus on unhealthy genes, inbreeding can actually be used to eliminate genetic conditions in dogs. Designer dog sellers like to say their pups have 'hybrid vigor,' but the unfortunate truth is that mixed breed dogs can actually inherit the worst traits of both parent breeds. The best option when buying a puppy is to find someone who knows their dogs' pedigrees inside and out and can match you with the most compatible puppy in the litter. Not someone who throws two cute dogs together, slaps a '-poo' on the mutt, and a couple extra zeroes on the price tag.
See also: What Makes a Good Dog Breeder?