Genetic Diseases of Purebred Dogs: The domestic dog we know today is the result of selective breeding over the last 14,000 years. Originally, dogs were bred for their working ability and for their "docile" nature. More recently, the trend has been to choose dogs for breeding based upon their physical beauty. As a result of this tendency and the practice of in-breeding, today's average "pure" breed of dog has a higher probability of inheriting a genetic disorder than a mixed breed does. Common genetic diseases include Canine Hip Dysplasia, epilepsy, blindness, deafness, auto immune diseases, bone disease, ear, nose, and eye structural problems, and blood clotting disorders. Buying a puppy from a pet store or a "roadside" dog dealer greatly increases your chances of getting a dog with a genetic disease, since many of these puppies come from puppy mills or situations under which the parent dogs are not screened for genetic disease. Purebreds without genetic disorders can be obtained from a very conscientious breeder who methodically removes defective stock from the breeding program. Responsible breeders must be chosen very carefully (see Hallmarks of a Responsible Breeder).

Genetic Diseases of Mixed Breed Dogs: By their very nature, mixed breed dogs usually have a larger "gene pool", and thus have a theoretically lower chance of genetic disease as a group than purebred dogs do. The exception to this would be the case in which two like breeds make up a mix (Border Collie and Rough Collie, for example who are both prone to Collie Eye Anomaly), or in the case of Hip Dysplasia, which is common in a large percentage of medium and large breeds. The chances of getting a mixed breed dog with an inherited disease are statistically decreased, but not entirely eliminated.

Temperament of purebred dogs: There are books that say that a puppy inherits 70% of its personality from its mother. Not only will she affect her puppies genetically, but she also affects them socially. If the mother is shy or aggressive, this is a major warning sign. Even if the mother does appear to have a good temperament, be sure to run a temperament test on a puppy before you select it. There is bound to be a bully in every litter, who may turn into a very dominant dog later on.

Temperament of Mixed Breed Dogs: In most cases, at least one parent of a mixed breed puppy is unknown, so testing the puppy individually for its temperament is very important.  If you can tell that a dog is at least one breed, count on it inheriting some of those breed characteristics (i.e. if it looks like a terrier, guess that it will probably like to dig and bark)

Socialization of purebred dogs: Your breeder should be offering puppies at about 7-8 weeks of age. If a puppy is separated from its mother and siblings too early (before the seventh week) it may have trouble relating to other dogs. If it is left with its mother and littermates past the twelfth week with little or no human contact, it will be unlikely to bond well to humans.)

Socialization of Mixed Breed Dogs: Often, a mixed breed puppy comes from a non-ideal situation. If the puppy was separated from its mother and siblings too early (before the seventh week) it may have trouble relating to other dogs. If it is left with its mother and littermates past the twelfth week with little or no human contact, it will be unlikely to bond well to humans. Again, testing the mixed breed dog individually for its temperament is very important.

Size of Mixed Breed Dogs: The size of a mixed breed puppy cannot be determined by its paw size! (Consider the relative paw sizes of Basset Hounds vs. Greyhounds.) If one parent is known, or the puppy looks like a recognizable breed, assume it will be as large as the largest breed it could possibly be. Female dogs stop growing at 18 months and male dogs stop growing at about 9 months; getting an older mixed breed puppy is the best guarantee on size.

Coat Type of Mixed Breed Dogs: Many puppies have a furrier coat as puppies than they will have as adults. Often they will shed this puppy coat somewhere around 6-8 months. Colors and markings in mixed breeds often change with this shedding. If you see wire-hair, short hair, or curly hair on a puppy, you can probably assume it is there to stay; fuzzy can turn into anything.