Pure Belgian Blue used to come from a relatively small population because it was a local breed. In recent years, its size has grown considerably and it has spread to several countries. It is also very important to consider blood relationship (inbreeding). The Belgian Blue breed also faces inbreeding in its population. The cause is found several decades ago, when there was widespread use of a few popular AI bulls
Inbreeding is the pairing of animals with a common ancestor. With corresponding genotypes, the probability of inbreeding is higher than with random mating. Usually inbreeding is harmful; it is accompanied by inbreeding depression. The result is a reduction in resilience and fertility, which result from the breakdown of harmful traits, such as in animals carrying inherited defects. Inbreeding increases the frequency of homozygous genotypes and thus also that of homozygous recessive genotypes.
Inbreeding does not cause defects, but it does result in increased homozygosity. Inbreeding can only reveal defects that were already present in the breed.
The rarer the defect, the greater the role of inbreeding. If the defect is rare, it will be confined to a single family and members of this family will have to merge to get two copies of the same gene, which could give these animals a defect. If the defect is frequent, it will occur in multiple families. Then it is possible for unrelated parents to have offspring with the defect without inbreeding being present.
It is therefore very important that pedigrees be complete. At least five recorded generations are needed to reliably calculate the inbreeding percentage of an individual animal (the inbreeding percentage is the sum of several common ancestors and not, for example, 1 common ancestor).
The following tables show which inbreeding coefficient is still acceptable and the coefficient at different mating possibilities.
Inbreeding is the pairing of animals with a common ancestor. With corresponding genotypes, the probability of inbreeding is higher than with random mating. Usually inbreeding is harmful; it is accompanied by inbreeding depression. The result is a reduction in resilience and fertility, which result from the breakdown of harmful traits, such as in animals carrying inherited defects. Inbreeding increases the frequency of homozygous genotypes and thus also that of homozygous recessive genotypes.
Inbreeding does not cause defects, but it does result in increased homozygosity. Inbreeding can only reveal defects that were already present in the breed.
The rarer the defect, the greater the role of inbreeding. If the defect is rare, it will be confined to a single family and members of this family will have to merge to get two copies of the same gene, which could give these animals a defect. If the defect is frequent, it will occur in multiple families. Then it is possible for unrelated parents to have offspring with the defect without inbreeding being present.
It is therefore very important that pedigrees be complete. At least five recorded generations are needed to reliably calculate the inbreeding percentage of an individual animal (the inbreeding percentage is the sum of several common ancestors and not, for example, 1 common ancestor).
The following tables show which inbreeding coefficient is still acceptable and the coefficient at different mating possibilities.
| Acceptable | Increased risk | To avoid | |
| Inbreeding coefficient | <3,125% | 3,125-6,25% | >6,25% |
| Inbreeding coefficient of different combinations | |
|---|---|
| Coupling between | Inbreeding coefficient |
| Parent X Offspring | 25% |
| Brother X Sister | 25% |
| Half-Brother X Half-Sister (= 1 common parent) | 12,5% |
| Grandfather X Granddaughter | 12,5% |
| Cousins (2 common grandparents) | 6,25% |
| Son X Granddaughter (2 common grandparents) | 6,25% |
| Grandson X Granddaughter (2 common grandparents) | 3,13% |