Understanding the Inbreeding Coefficient (COI)
The inbreeding coefficient (COI) is a single number that summarises how related an animal’s parents are. It is one of the most important figures in a responsible breeding program. Here is what it means and how to manage it.
What COI actually measures
COI is the probability that two copies of a gene in an individual are identical because they were inherited from a common ancestor on both sides of the pedigree. It is expressed as a percentage from 0% (no shared ancestors in the pedigree examined) upward.
Wright’s coefficient is the classic formula: COI = Σ (½)^(n1+n2+1) × (1 + FA), summed over every common ancestor, where n1 and n2 are the number of generations from each parent back to that ancestor and FA is that ancestor’s own COI.
What is a healthy COI?
Always compare COI over the same number of generations — a 5% COI over 10 generations is very different from 5% over 4 generations.
- Under 5% — low; comparable to breeding unrelated animals within a breed.
- 5–10% — moderate line-breeding; common and generally acceptable.
- 10–25% — high; equivalent to half-sibling or grandparent matings. Proceed only with strong justification.
- Over 25% — very high; equivalent to full-sibling or parent–offspring. Strongly discouraged.
Why high COI matters
Higher inbreeding increases the chance that harmful recessive genes pair up, raising the risk of inherited disease. It is also linked to "inbreeding depression": reduced fertility, smaller litters, weaker immune function, and shorter lifespan.
How to lower COI
- Introduce an unrelated (outcross) animal to the line.
- Track COI before every planned mating rather than after.
- Prioritise genetic diversity alongside type and health testing.
- Keep multi-generation digital pedigrees so calculations use deep data, not just parents.
Frequently asked questions
Is a 0% COI always best?
Not necessarily. A very low COI maximises diversity, but some controlled line-breeding is used to fix desirable, healthy traits. The goal is a low, managed COI combined with health testing — not zero at any cost.
How many generations are needed for an accurate COI?
At least 4–5 generations. A COI calculated on only parents and grandparents will underestimate the true value because it misses shared ancestors deeper in the tree.