The Breeding Toolbox: Linebreeding

Linebreeding and inbreeding are closely related.  In fact, linebreeding could be considered a subcategory of inbreeding.

Because the two tools are so similar, there is quite a bit of confusion about the differences between linebreeding and inbreeding.  Simply put, linebreeding is a method of increasing the influence of one or more animals on a genetic pool while attempting to keep the inbreeding coefficient relatively low.  For example, mating close relatives such as full brothers and sisters or parents and offspring would not be considered in a linebreeding program.  However, matings between more distant relatives, such as between cousins or half-siblings, would be accepted.

In the pedigree below, using the bloodline of imaginary horses that we created in the last part of the series, we have an example of linebreeding to Tornado and Cloud.  Gold Dust and Lightning are cousins.  As a result of this mating, Cibola has an inbreeding coefficient of 0.0625.

How It Works

Linebreeding works exactly the same as inbreeding, by pairing like genes from the same ancestor.  However, because the relationships are distant, the frequency of like genes is greatly diminished.  Cibola, for example, has only a 6.25% chance of receiving duplicate genes from Tornado and Cloud.

Applications

Linebreeding is typically used for the same reasons as inbreeding:

• Uniformity.
• The creation of distinct bloodlines for the purpose of crossbreeding.
• To force genetic defects to the surface for culling.

However, because inbreeding can be so risky, linebreeding frequently serves as a compromise solution.  Breeders sacrifice some of the powerful effects of inbreeding to avoid some of its severe shortcomings.

In linebreeding, many pet and livestock breeders choose a maximum acceptable level of inbreeding.  This level is high enough that the breeder can make substantial progress in shaping his bloodline, but low enough to maintain a degree of hybrid vigor.  The exact level varies with each breeder and breeding program, but 0.06 is a commonly accepted maximum inbreeding coefficient.

Pitfalls

The pitfalls of linebreeding are very similar to those of inbreeding:

• Hybrid vigor can dwindle if the breeder does not pay strict attention to the inbreeding coefficient.
• Undesirable genetic traits can become fixed into the bloodline if the breeder does not exercise good culling practices.

True, the risks of linebreeding are far less than those of the closest inbreeding.  Still, this tool is most effective in the hands of a breeder with definite, realistic goals and a willingness to cull ruthlessly enough to meet those goals.