Inbreeding is a breeding technique which pairs closely related animals, such as father-daughter, brother-sister, or cousins. However, the strict scientific definition of inbreeding counts all relationships that are duplicated on both sides of the pedigree. Animal breeders by tradition consider distantly related crosses to be linebreeding rather than inbreeding, but the scientific basis of this concept is not obvious. Linebreeding is in essence a moderate form of inbreeding.
The theoretical basis of inbreeding is to produce stock of consistent excellence through creation of homozygous type.
Most breeders prefer to avoid inbreeding. Instead, they keep the overall relationship to common dogs rather high by using them several times further back in the pedigree. Inbreeding increases the probability that any gene will be homozygous by descent. Of course, genes of excellence will be passed along as well as genes of faults. Among some breeding stocks, inbreeding leads to reproductive problems (infertility) and reduces disease resistance. It is currently estimated that there are over 370 genetic diseases found in the domestic dog, with 5 to new 10 disorders identified each year. More than 70% of these disorders are caused either by autosomal or sex-linked recessive genes, or have been positively associated with inbreeding, especially in breeds that originated from a limited genetic pool.
A common misconception is that inbreeding causes high-strung, nervous and aggressive dogs. Because two individuals are closely related does not mean that their offspring are going to be unsound. It is the genetic potential in the background of the pair which determines the outcome. A fundamentally sound strain remains fundamentally sound. One which has some unsuitable dogs in his inbreeding program is likely to have problems.
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After having linebred for three or four generations, most breeders have found from experience that it is wise to bring a new blood. The use of a stud from a totally different bloodline may be considered. This produces and outcrossed litter and "reshuffles" the genes that have tended to become fixed, in a more or less predictable manner, through previous linebreeding.
Many times, particularly with an overly refined bitch, an outcross will give surprisingly good results. An improvement in the health and vigor of the resulting puppies is apparent from the time they are born. This process is known as "nicking". While the litter will sometimes lack uniformity, nevertheless some really good show dogs have been produced in this manner.
When two strains have nicked successfully, other crosses between them may work as well. Puppies from such matings usually are bred back into one of the two strains, thereby providing a basis for a new line.
One final method is to breed a dog and a bitch who are both of mixed ancestry. Neither has a linebred background. When using this approach, it is essential that one has a definite goal in mind. One dog may carry an attribute or quality totally lacking in the other. However, the method of breeding strengths to weaknesses in hopes that the strengths will win out sometimes is disappointing—too often it is the weaknesses which win out producing puppies of inferior quality.