Understanding and Using the Coefficient of Inbreeding
Before we get to the inbreeding coefficient, it’s helpful to be reminded of the basic operation of genes. A gene is a strand of DNA that exists in a living cell and determines the form and function of an animal. Genes are passed from one generation to the next through the act of breeding. Each animal gets its genes in equal parts from its mother and father. So, for example, when you breed a sow and boar, the sow contributes a hair color gene and a boar contributes a hair color gene, and together this pair of genes determines the hair color in the offspring they produce. If the sow and boar share a common ancestor (for example, the same grandmother) there is a chance —a mathematical probability— that the sow and boar will each be passing to their offspring the identical hair color gene that originated in that common ancestor, the grandmother.
The inbreeding coefficient is a measure of the probability that the pair of genes will be identical, having originated with a common ancestor. The coefficient of inbreeding, or COI as it’s known, is a number between 0% and 100%. If the COI of an animal is 0%, then there is no probability that the animals share a common ancestor that contributed a common gene to their offspring. If the probability is 100% (a condition that almost never occurs), then it is a highly inbred animal and there is an absolute certainty that the same genes from a common ancestor will be paired in the offspring.
So, why should you care about the COI? Because as a general rule, the greater the inbreeding in an animal, the more likely it is that harmful genes will be expressed. When genes form pairs in the offspring, when each gene in the pair is at least slightly different, it tends to dampen the expression of negative genetic characteristics. But when the paired genes are identical, it is more probable that negative genetic characteristics will be expressed. This is what scientists called inbreeding depression, a condition that reduces the general fitness of an animal population.
It may be useful to think about the opposite effect. We all know that some animal breeders choose to cross unrelated breeds and produce superior offspring, for example, crossing Red Wattles with Durocs to produce excellent meat hogs. This phenomenon is called hybrid vigor, and it shows that genetic differences between mates reduce the weaknesses and enhance the strengths of the breeds.
If an individual red wattle has a high COI, it doesn’t mean it is an inferior animal. Remember, the COI is merely a probability that the genetic pair in an animal originated in the same common ancestor. In reality, some animals are lucky and they have superior genetics despite inbreeding. But, when you breed a boar and a sow with the knowledge that their young will have a high COI, you increase the chance that the young will express the harmful elements in their genetics. All things being equal, you’d rather plan a mating between sow and boar that results in a lower COI. You can use the Pedigree Analysis function on this website to predict the COI of offspring that would be produced if you mated two animals. You can access the Pedigree Analysis functions from the Members Area of the site. If you haven’t set up your Members Area account yet, please see our Members Area Info page to find out how.
What COI should you try to achieve when you breed animals? There is no hard and fast rule. Livestock experts have said that a COI of .0625 (6.25%) or lower is routinely acceptable. When breeding endangered species with a small population that offers limited pairing choices, zoo experts say that .100 (10%) may be acceptable. For Red Wattles, you may want to set a as a goal to produce offspring with a COI of .0625 (6.25%) or less.