Coefficient of inbreeding (COI): what it means and how to read it
COI is the number breeders check for every planned mating. Here is what it actually measures, how it is calculated and why the same pairing can come out at 3% one time and 9% the next.
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In short
- The coefficient of inbreeding (COI) is the probability that an offspring inherits the same copy of a gene from a common ancestor through both parents.
- It is calculated from both parents’ pedigrees using Wright’s method. Deeper and more complete pedigrees give more accurate – and usually higher – results.
- As a guide: parent × offspring or full siblings 25%, half-siblings or grandparent × grandchild 12.5%, first cousins 6.25%.
- There is no universally safe threshold. Always compare values calculated over the same number of generations and follow your club’s rules.
What COI measures
An animal carries two copies of most genes – one from its sire and one from its dam. When sire and dam share an ancestor, the offspring can inherit both copies from that ancestor. Such copies are called identical by descent. COI is the probability that this happens at any given position in the genome.
A COI of 0% means the parents have no common ancestor within the pedigree depth used. COI belongs to the offspring: it describes how related the parents are, not the parents themselves. That is why it can be calculated for a mating you are only planning.
Why COI matters
The more closely related the parents, the higher the chance that recessive inherited disorders hidden in the parents show up in the offspring. Persistently high inbreeding in a population is also linked to lower fertility, smaller litters and reduced vitality – known as inbreeding depression.
COI is one tool for responsible planning, not the only one. Health and genetic tests of both parents, conformation and temperament matter just as much.
How COI is calculated
The most widely used approach is Wright’s method. For every common ancestor, all paths leading from the sire through that ancestor to the dam are found. Each path contributes (1/2)ⁿ, where n is the number of animals on the path including the sire, the dam and the common ancestor. If the common ancestor is itself inbred, its contribution is increased by its own COI. The contributions of all paths are added up.
Typical values for common matings
| Mating | Offspring COI |
|---|---|
| Parent × offspring | 25% |
| Full siblings | 25% |
| Half-siblings | 12.5% |
| Grandparent × grandchild | 12.5% |
| Uncle or aunt × nephew or niece | 12.5% |
| First cousins | 6.25% |
| Half-uncle × half-niece | 6.25% |
Real pedigrees often contain several common ancestors at once, so the result is usually higher than in the table.
Why the result depends on the number of generations
Each extra generation can reveal common ancestors that a shorter pedigree does not show. A 4-generation COI is therefore almost always lower than a 10-generation COI for the same mating. A number without its depth says little: 5% over three generations and 5% over twelve generations mean very different levels of relatedness.
What COI value is acceptable?
There is no universal threshold. It depends on the breed – breeds with a small gene pool often show a high COI even for matings that are unrelated in recent generations –, on the number of generations and on your breed club’s rules. A practical approach:
- Find out what limits or recommendations your club and breed advisor use.
- Compare the planned mating with typical values in the breed at the same pedigree depth.
- When choosing between comparable partners, prefer the mating with the lower COI.
- Do not rely on COI alone – take both parents’ health and genetic tests into account.
How to calculate COI in Breedio
In the Genetics module, choose the sire and dam – including a planned mating before it happens – and select the number of generations. Breedio calculates COI with Wright’s method from both parents’ pedigrees. It matches ancestors by registration numbers and names, so it recognises the same animal in pedigrees written differently.
Accuracy depends on how complete the pedigrees are. Add missing ancestors manually, by importing a photo of the pedigree certificate, or from a PawPeds or BreedArchive link.
Sources
- Wright, S. (1922): Coefficients of inbreeding and relationship. The American Naturalist 56, 330–338.
Related
- Breedio feature: Genetics & testing
- Frequently asked questions