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LINE BREEDING OR INBREEDING ? Written by Darren Todd
What is Line breeding? Cattle breeders wishing to concentrate the genes of outstanding individuals have occasionally mated closely related animals. For example, the late Canadian master breeder, Roy Ormiston, would often mate descendants of his iconic white cow family together. One such mating produced the famous bull Roybrook Tempo, whose sire and paternal grandsire were maternal bothers. Tempo was, therefore, line bred to their dam, Roybrook Model Lass (see Fig. 1). He became an outstanding sire of type and in particular stature and size.
So was this also inbreeding? In genetic terms absolutely.
Tempo has an inbreeding coefficient of 8%, of which 3% alone was contributed by Roybrook Model Lass appearing on both sides of his pedigree. Essentially, all pedigree cattle breeding involves matings of at least distant relatives, which is not necessarily undesirable. However in genetic terms,
matings resulting in predicted inbreeding coefficients above 6.25% are considered undesirable.
Does Tempo's inbreeding coefficient suggest he was risky to use? Not necessarily, Tempo could be mated quite sensibly with cows which were not closely related to him, for example the prominent Elevation or Chief bloodlines.
The inbreeding coefficient of the predicted mating is important rather than the coefficient of the individual parents. Two highly inbred, but unrelated animals can be mated and produce offspring with a low inbreeding coefficient.
So the difference between Line breeding and Inbreeding is...
Simply one of perception. Line breeding is associated with a positive outcome of mating close relatives, whereas inbreeding is seen in more negative terms, but they describe exactly the same genetic concept.
What are the potential negatives associated with Line Breeding and inbreeding?
Undesirable recessive genes can be concentrated, or in genetic terms made homozygous, by the line breeding process. BLAD and CVM are the two classic examples of this in the Holstein population.
Osborndale Ivanhoe and Carlin M Ivanhoe Bell were the prominent AI bulls which spread these alleles (versions of genes) in the Holstein population worldwide. Both bulls had desirable breeding patterns and so were heavily used. Inevitably their descendants were eventually mated together and the two alleles were paired in some animals which were thus homozygous, resulting in calf deaths.
Only at this stage did research identify Ivanhoe and Bell as carriers. This is the key problem, in that most of these deleterious alleles are not identified until homozygous animals appear in the population. Although nowadays genomic selection has speeded up the process of identifying probable carriers.
The white cow family, with Roybrook Model lass in the middle and the white cow herself, Balsam Brae Pluto Sovereign, at the right.
How many bulls carry undesirable recessive genes? Potentially all of them and in every breed, as it is a natural consequence of evolution that animals carry these alleles which are largely harmless when only one copy (heterozygous) is present. Most recessives do not have catastrophic effects and will typically only have a small impact on a given trait such as fertility. Some recessives such as BLAD, CVM and more recently Cholesterol Deficiency
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