Controlling Inbreeding

Author: Mark Mortimer

Article Type: General Articles

It is a deliberate policy at Centre Plus to keep the Average inbreeding coefficient as low as possible. Simm’s [1] describes the inbreeding coefficient as the probability that two alleles at any locus are identical by decent. For example mating two half sibs together would produce an animal that has a 12.5% chance of inheriting the same genes from both parents.

Many studs use inbreeding (Line breeding) as a means of increasing the frequency of favorable genes and locking in a type. But this type of breeding comes at a cost. The two main drawbacks of inbreeding are the reduction of genetic variation and a decline in performance of traits linked to fitness known as inbreeding depression.

Variation in a flock is one of the key factors influencing the speed of genetic gain. The more genetic variation a trait has the larger the selection differential and the faster the gain. This is usually where some one will point out that two much variation is a bad thing (e.g. the variation in micron when trying to put together a wool clip for sale). But it must be kept in mind that we are talking about genetic variation, not the phenotypic (the full range of measurements from a drop of animals) variation. For example Fiber Diameter at Centre Plus has a heritability of 0.67 [2], which means that genes contribute 67% of the observed variation and environmental factors make up the rest. Influencing the genetic variation does not mean a large change in actual variation particularly with traits with a low heritability like Greasy fleece weight at 0.34 [2].

“Inbreeding depression is thought to be the result of an increase in frequency of recessive genes which adversely affect those characters associated with survival and overall fitness” [1].

Some examples and effects of every 1% increase in inbreeding.

1. Greasy fleece weight –0.017Kg.
2. Weaning Weight -0.111Kg.
3. Ewes Lambing/Ewes Joined –0.14
4. Lamb survival (per 1% increase in inbreeding of Lamb) –0.028
5. Lamb survival (per 1% increase in inbreeding of Dam) –0.012

Reference
1. Geoff Simm. 1998. Genetic Improvement of Cattle and Sheep. Farming Press, Miller Freeman UK Ltd.
2. Daniel J. Brown. 2006. Genetic aspects of greasy wool colour assessments in Merino sheep. International Journal of Sheep and Wool Science Volume 54, Issue 3 2006 Article 1