By Sarah Karr,
Roving Acres Farm, Pierpont, Ohio
If you have been on the JSBA pedigree search lately, you may have noticed some new letters after the sheep’s other relevant information. Recently, the decision was made to include COI in the publicly available pedigree search so that everyone has access to that information. While this information has long been available to our registrar, there was initially some concern that uninformed use of COI in breeding decisions could be harmful to the long term viability of the breed. It was decided that the more information the better when it comes to breeding decisions and instead of restricting access to COI percentages we are aiming to make membership more informed about their use.
What is COI?
COI, or Coefficient of Inbreeding, is a measure of how closely related an individual’s parents were. It is expressed as a percentage which represents the probably that alleles at any given locus are identical due to being inherited from the same ancestor. This means that a COI of 20% represents a 20% likelihood of any given gene in an individual being identical from both parents due to relatedness. This calculation is generally based on the number of common ancestors present in a recorded pedigree and can change depending on how many generations back you look. In some species a genetic COI based on measured genetic similarities can be calculated for greater accuracy but to my knowledge this has not been done in sheep before.
What about the numbers?
The lower the animal’s COI, the less inbreeding present in the animal’s pedigree. A breeding between a dam and son or sire and daughter or full siblings, with otherwise low inbreeding, has an expected COI of around 25%. A breeding between grandparent and grandchild or half siblings has an expected COI of around 12.5%. These numbers will vary depending on the amount of inbreeding in previous generations. When related animals with already high COIs are bred together, they will produce offspring with higher COIs than expected based solely on relatedness.
What does this have to do with flock health?
When an individual has a higher number of identical genes from both parents, they have a higher likelihood of expressing recessive traits. These traits could be good, bad, or neutral but they are more likely to be expressed. A sheep with high levels of inbreeding is much more likely to express any recessive trait carried in its lines than a sheep with a very low level of inbreeding. This doesn’t mean that pairing two unrelated sheep will never express recessive traits or that highly inbred sheep must express recessive traits, it simply changes the probability if all else is equal.
In species like dogs and even more commercially successful breeds of sheep there are genetic tests for recessive traits that may be harmful to potential offspring allowing people to inbreed heavily while still avoiding testable traits. The problem is that there simply is not a test for every issue and the combined affects of reduced overall genetic diversity can result in inbreeding depression.
Inbreeding makes my sheep depressed?
Inbreeding depression is the reduced survival and fertility of inbred offspring. This happens in both wild populations and domestic species with increased COI. Effects can begin with COIs as low as 5%. While not directly studied in sheep, in general with every 10% increase in COI can cause a decrease in fertility, production, lifespan, and health across a wide variety of species. Even when you can’t point directly to a single recessive trait causing issues, having varied alleles is important for producing the healthiest offspring possible.
Then why inbreed at all?
Every purebred animal is built on some level of inbreeding. The creation of a breed purposefully creates what is called a bottleneck. In order to create a population that breeds true that group must be removed from the population of that species at large to focus their reproduction on specific traits. Within those breeds are lines, which then isolate populations even more. Inbreeding, often called linebreeding, is critical for creating consistency within lines.
When you see a lamb and go “that must be from Farmer Bob, I can tell his sheep anywhere”, you can thank inbreeding! Increasing sameness in the genetic pool doesn’t just increase the likelihood of bad traits coming to light, it also increases the likelihood that the offspring will express all of those good traits you love in its parents. Inbreeding is a tool savvy breeders use to consistently get the traits they want, generation after generation.
My sheep has a high COI, should I be worried?
The key with COI is to monitor it among the whole population and worry less about it on the individual level. COI is a measure of risk of recessive traits being expressed, not a direct prediction of health. The vast majority of high COI individuals will live perfectly healthy, productive lives. It is also relatively easy to reduce the COI in your flock within one generation by simply bringing in an unrelated ram. Breeding two sheep with 30% COIs together can create 0% COI offspring if the sheep are not related to eachother.
The problem comes when the entire population is highly related and you can no longer find unrelated breeding pairs. In those cases it may become necessary to outcross to other breeds to reduce the level of relatedness in the overall population. However, even when the entire species is highly inbred it can take thousands of years for the effects to be felt, as is the case with cheetahs which have spent the last 10,000 years so inbred you can do skin grafts between unrelated individuals without risk of rejection.
Takeaways
JSBA’s open studbook and the ability to bring in sheep that visually appear to have the traits we select for as Jacob breeders helps safeguard our population from some of these inbreeding risks. Watching the COI levels of your flock can help you make responsible breeding decisions and balance the benefits of consistency with the risk of inbreeding depression.