6.15: What rate of inbreeding is acceptable?

The rate of inbreeding provides an indication of the expected increase in problems due to increased homozygosity. It provides an indication of the risk of getting into trouble. A risk means that it could be better or it could be even worse. As a rule of thumb, the FAO advises to restrict the rate of inbreeding to below 1%, preferably below 0.5%. A rate of inbreeding of 1% indicates that the increase in homozygosity will be 1% per generation. But there is no one who can predict what the consequences will be for the increase in problems due to homozygosity. Because not all genes have equal effect, and also not all alleles within a gene have equal effect.

The FAO advise is related to risk management, not to insurance. A rate of inbreeding above 1% increases the risk that the population in the long run will not survive.

Assuming balanced breeding, a rate of inbreeding of 1% means that at least 25 males and 25 females need to be used for breeding. Likewise, a rate of inbreeding of 0.5% means 50 males and 50 females. If non-balanced breeding is applied, the numbers of breeding males and females need to be chosen such that the rate of inbreeding is restricted to less than 1 (or 0.5)%. Depending on the animal species, this will be a realistic option or not. Consequences of number of breeding animals on the rate of inbreeding is essential to keep in mind when making breeding plans.