Inbreeding occurs when a mating was made with a relative or the parents shared common ancestors. The closer to the common ancestors and the more common ancestors, the stronger the inbreeding.
The most severe inbreeding occurs in a sibling to sibling mating or offspring to their parents. These matings commonly occur in an effort to preserve features of a breed or line within a breed and is referred to as “line breeding”. The degree of inbreeding can be mathematically expressed as an inbreeding coefficient. The inbreeding coefficient of an individual is defined as the probability that any two homologous alleles (same forms of the genetic locus) are identical by descent. That is, they were transferred from an ancestor to the current generation.
Inbreeding often occurs the deeper you trace a pedigree. It is almost impossible to avoid inbreeding in a limited population, especially when the population has experienced a bottle neck. Severe inbreeding could result in shorter lives and problems of fitness including hip dysplasia. The level of inbreeding has continuously accumulated in Labrador retriever dogs over the past 40 years. Levels of inbreeding can be seen in the accompanying paper.