Inbreeding can take place in two different ways in a population: 1) breeders mate intentionally a sire and dam that are more related to each other than on average in population. This is called voluntary inbreeding. 2) breeders have to mate sires and dams that are related due to the fact that all animals in the population are related to each other. This is called constrained inbreeding and will be the main type of inbreeding we will discuss when monitoring populations. Constrained inbreeding is caused by the limited size of closed breeding populations as is the case in many breeds.
In each generation further in the pedigree the number of ancestors increases exponently. E.g. in generation 10 an animal has 210 = 1024 ancestors. In most breeds in the period the ancestors of generation 10 lived, less than 1024 animals were used for breeding. Thus further in de pedigree the same animals pop up in the pedigree of the sire and the dam: they are related and therefore their offspring becomes inbred. This emphasizes that the deeper the pedigree, the better the relationship between sire and dam can be established. As (constrained) inbreeding is indeed an issue in dog breeding, most examples we will present, will originate from dog breeds.