6.13.1 Mating systems and inbreeding

It is also possible to perform inbreeding on purpose. For example, by mating a father to his daughter and then also to his granddaughter. This is called line breeding. Some breeders perform line breeding because they want to fix the positive alleles of a certain superior male. In theory this is not a bad idea. Inbreeding increases homozygosity, so that also involves the positive alleles. Unfortunately there are two main reasons to avoid line breeding. First of all with line breeding animals become inbred, and thus homozygous, for many and eventually most of the alleles from that superior ancestor animal. However, not all of his alleles are equally desirable. He will be carrier of a number of recessive disorders that may become homozygous in the inbred descendants. The superior animal does not suffer from these disorders because he is heterozygous, however his inbred, and thus homozygous, descendants will be. Second, line breeding (as any inbreeding) has a serious decreasing effect on the genetic variation in the population if applied by most breeders. This may have consequences for the ability of the population to adapt to a changing environment in the future. The increased inbreeding level because of mating decisions can be reversed by dropping the mating restrictions and introducing random mating. This is illustrated in figure 11c.

The examples in figure 11 show that the rate of inbreeding in the long run depends on the average additive genetic relationship between the animals in the population. You can try to avoid inbreeding or increase inbreeding by adjusting the mating strategies. However, in the end the rate of inbreeding will be determined by the average genetic relatedness between the animals.