Purebred dogs come from genetically closed populations. Purebred dogs are usually very genetically similar to one another, and the rarer the breed, the more this holds true. Close breeding means that purebred dogs are very similar in appearance, but it can also lead to health problems. Not all purebred dogs are of the same quality; here's some more information on how to select a top-quality purebred puppy, or how to breed your own dogs.
Understanding Genetics
Gregor Mendel, a scientist who lived from 1822-1884, developed the basic laws of genetics by cross-breeding pea plants to produce different, desired qualities. These principles also apply to dogs. Traits result from the interaction of two corresponding alleles (a dog gets one from each parent). Some alleles are "recessive", which means they will only lead to their corresponding trait if a dog gets both of them, one from each parent (if the dog gets a "dominant" allele from either of his parents, the dominant allele will lead to its trait). Specific, pure breeds that had evolved in a closed group for a long amount of time are more likely to carry two recessive alleles that produce a trait that can be detrimental to their health. Dog Types
There are many different types of dog breeds, and breeding dogs produces more than just physical traits. If you're thinking of breeding dogs, it's a good idea to consider the background of the breed first, as each breed may have been specifically bred to to serve a different purpose. Some dogs were bred to sit in laps; others were bred to hunt and work; some were bred as companions; others need a very active lifestyle.
Remember, an Australian shepherd will retain his tendency to herd, even if you don't have any sheep. Dalmatians will have incredible stamina and won't be satisfied with the short walk that may wear out a dog of another breed. Rottweilers were bred to drive cattle, and will continue to have strong muscles and powerful jaws even if you live in a city apartment. Health Problems
Another thing to consider, if you'd like to breed dogs, is the fact that many dog breeds suffer from inherited health problems. Hip dysplasia, for instance, is particularly common to many breeds. If you're going to breed purebred dogs, make sure both parents are healthy and have been screened for breed-specific health problems; this can reduce your chances of producing puppies with health problems.
This is particularly important if you're breeding rare breeds of dogs. Remember, the more rare the breed, the more closely bred the dogs will be; the more closely bred the dogs, the more likely they are to have health problems.
Personality
Even within breeds, different dogs will have different sets of skills and personalities, depending on the skills and personalities of their parents, grandparents, and so forth. If you'd like to breed dogs for very specific purposes, look for puppies descended from ancestors who excelled at those tasks. Designer Breeds
One of the newest trends in breeding dogs is the creation of designer breeds, such as the Labradoodle (a cross between a Poodle and a Labrador Retriever). Designer dogs are created by breeding two purebred dogs together to create puppies that share traits common to both breeds of dogs. However, designer breeds are not pure breeds; if you breed two Labradoodles together, for instance, the puppies might look very different from their parents.