To be an effective breeder, you should have a basic understanding of the science of genetics. Everything about your prospective puppies' health, soundness, looks, and temperament will be determined by the genes passed on by their parents, and by their parents before them. Therefore, the selection of a mating pair should not be made solely on the basis of the dog's or bitch's looks (or temperament or soundness, etc.), but should be based on an understanding of how the animal's genes contributed to its looks and of how those genes are passed on and expressed. That is why it is essential to study the pedigrees of your mating pair. The more knowledge you have as you make your selection, the more likely you are to produce a litter with the qualities you desire.
You should also be well-versed in the genetic problems that affect your breed. Genetic defects can occur in any breed and can affect any system in the body. Some genetic diseases may occur in many breeds; others occur in only one or a few breeds. The following is a brief explanation of how genetic defects may be inherited and expressed.
Diseases that follow a dominant pattern of inheritance need only one abnormal gene. That is, if only one parent is affected, the condition will show up in each successive generation. Some individuals may be only mildly affected with the condition, making it difficult to detect. In such cases, the condition can mistakenly be thought to skip generations.
Diseases that follow a recessive pattern of inheritance occur in homozygous individuals, meaning dogs with two abnormal genes. Dogs with one mutant and one normal gene are heterozygous, and they are carriers of the condition. They appear normal but can pass the abnormal gene to their offspring. Recessive mutant genes can be passed through many generations before emerging in the offspring of two dogs that carry the same genetic mutation.
Polygenic disorders result from the cumulative action of a number of different genes. The exact number of genes involved and their individual functions are difficult to determine, and the pattern of inheritance tends to vary from family to family. Polygenic inheritance can sometimes mimic either dominant or recessive inheritance, and this feature may lead to erroneous conclusions regarding the type of underlying genetic abnormality.
Chromosomal anomalies -- defects in chromosome number and structure -- can also cause genetic diseases. Dogs normally have 39 pairs of chromosomes on which genes are located. Major abnormalities in chromosome number and structure can produce serious defects.
Whether you inbreed, line breed, or outcross may have an effect on the incidents of genetic disease in the offspring. Inbreeding is the mating of two individuals that are related through one or more common ancestors. The closest form of inbreeding involves parent-child and brother-sister mating's. Line breeding, a form of inbreeding, usually involves mating more distantly related dogs. Typically, these mating's involve, cousins (with one or more grandparents in common), nephew-aunt, great granddaughter-great grandson, grandfather-granddaughter, grandmother-grandson, etc. The rate of polygenic and recessively inherited diseases tends to increase with inbreeding, because the chance that the two animals carry the same mutation is greater when the dogs are related. Out crossing is the mating of two dogs of the same breed that are otherwise unrelated.
Genetic Glossary of Terms Link