Inbreeding versus linebreeding – What is the difference?
Michiel M Scholtz1 & Michael D MacNeil2
1 ARC-Animal Production, Irene, South Africa; 2Delta G, Miles City, Montana, USA
Inbreeding results from the mating of related animals. Everyone realizes that the mating of a father with his daughter or the mating of a brother and sister definitely produces inbreeding. But what about the mating of a grandfather with his granddaughter, a nephew with his niece, or two cousins, is that inbreeding? In humans, the latter is commonly regarded as inbreeding and is normally forbidden.
Linebreeding can be thought of as inbreeding with a purpose. That purpose is to increase the genetic relationship to an outstanding individual while at the same time avoiding the accumulation of additional inbreeding to less worthy ancestors. Implied here are some genetic principles that merit clarification.
The number of Ankole cattle in South Africa is limited, and until recently, the genetic pool has been small. It is therefore unavoidable that inbreeding will occur. In a breed like the Ankole in South Africa, inbreeding will be the result of the limited number of bulls available that are not related to the females. It is therefore important that Ankole breeders understand both inbreeding and linebreeding.
The primary effect of inbreeding is that it increases the chance that an animal will receive the same allele of a gene from both parents. This will reduce the degree of heterozygosity (the presence of different alleles at particular gene loci) in the population and increase the relationships between animals to more than 50%. Inbreeding will therefore lead to an increase in homozygosity (having the same alleles at particular gene loci), which is the easiest way to fix certain alleles in the population. This will lead to uniformity, but also increases the chances to fix undesirable genes, such as skew face, hypoplasia and laminitis (outgrowth of hooves). This was the basis for which inbreeding was used in the formation of many breeds in the past.
Inbreeding depression is the reduction in the performance of inbred animals. This reduction is more subtle and more important than the occurrence of adverse recessive traits that are influenced by few genes and that are usually associated with inbreeding. It is known that inbred animals do not adapt as well to changing conditions, and their reproduction and production are also likely to be lower, compared to contemporary animals that are not inbred. Inbreeding should thus be avoided, unless it is done with a specific aim in mind.
The pedigree diagram in Figure 1 illustrates the mating of a sire to one of his daughters. Numerically, the relationship of the sire to his daughter is 0.50. The relationship of the sire to the progeny is 0.75, and the inbreeding of the progeny is 0.25, which is one-half the relationship of the progeny’s parents.
To be successful in a linebreeding program, a breeder must also understand the principles of inbreeding and relationship. Of primary importance in a linebreeding program is the selection of the “outstanding” individual that will serve as its foundation. Finally, success in a linebreeding program entails also patience, persistence and good luck.
It should be recognized that linebreeding also increases risk of failure in the breeding program. This risk is present because the chosen outstanding individual might also be a carrier of a recessive allele that is hugely undesirable. Linebreeding can also be focused on an individual whose genotype proves not to be in demand going forward. In the case of the Ankole, this may be the shape or length of the horns. The inbreeding that accompanies any linebreeding program might also yield inefficiencies in production that prove to be too costly. As a rule of thumb, the outstanding individual that is chosen as the basis of a linebreeding programme should be a truly elite individual within its breed.
A successful linebreeding program requires patience and persistence as it entail several generations of breeding. With a nine month gestation period, being able to first bear offspring at two to three years of age, and producing less than one offspring per female per year, cattle have a long generation interval. Modern assisted reproduction technologies like artificial insemination, embryo transfer, and juvenile follicle aspiration may hasten the program along somewhat. However, breeders considering a linebreeding program would be well advised to think in terms of at least a 10-year planning horizon.
When related individuals are mated, as in a linebreeding program, their progeny are expected to have alleles that are identical for more genes than if the parents were not related. If inbreeding and relationships are intertwined, why is inbreeding to be avoided when a degree of relationship is desired? Alleles with detrimental effects tend to be masked by an alternative allele of the same gene (i.e., the detrimental alleles tend to be recessive). Thus, the detrimental alleles only become apparent when they are identical on both chromosomes.
In South Africa, Ankole breeding programs may be limited by the lack of good quality unrelated bulls. In this situation it is important to take note of the relationship coefficients for prospective matings. There are computer programs that will alert the breeder if a certain mating will result in a level of inbreeding that is generally unacceptable. These programs may be less useful in a linebreeding program because the level of inbreeding that is generally unacceptable may be deemed acceptable if it arises from relationship to the outstanding foundation animal. To advertise a linebred animal, the “outstanding” foundation animal should also be identified. Irrespective of the breeding strategy that is used, it is important to accurately record pedigree data; especially in a breed with small numbers of animals, like the Ankole in South Africa.