Coefficient of Inbreeding (COI) Calculator
Estimate the expected inbreeding coefficient (F) of offspring from known parental relatedness. Use a preset relationship or enter a custom coefficient of relationship (r).
Note: If you enter a custom r, it overrides the preset. For non-inbred parents, the calculator uses F = r / 2.
What this inbreeding calculator does
This tool estimates the coefficient of inbreeding (COI), written as F. COI is the probability that two copies of a gene in offspring are identical by descent from a shared ancestor. In practical terms, a higher COI means more genetic similarity inherited from both sides of the pedigree.
Breeders, conservation programs, and population geneticists often use COI to monitor mating plans and reduce avoidable genetic risk over time.
How to use the calculator
- Select a parental relationship from the preset list, or enter a custom coefficient of relationship r.
- Click Calculate.
- Read the estimated COI, the equivalent relationship percentage, and the qualitative risk band.
Quick interpretation guide
- 0% COI: No expected inbreeding from known shared ancestry.
- 0.01%–3.13%: Low.
- 3.13%–6.25%: Moderate.
- 6.25%–12.5%: High.
- Above 12.5%: Very high.
The math behind the estimate
When parents are not themselves inbred and you know their relationship coefficient r, the expected offspring inbreeding coefficient is:
F = r / 2
Example: If parental relatedness is r = 0.125, then offspring COI is F = 0.0625 (6.25%).
In full pedigree analysis with shared ancestors, geneticists use path-based formulas such as:
Fx = Σ (1/2)(n1+n2+1)(1+Fa)
where each path passes through a common ancestor a, and n1 and n2 are generations from each parent to that ancestor.
Why COI matters in breeding programs
Tracking COI helps maintain genetic diversity. As diversity shrinks, populations may see more inherited disorders, lower fertility, weaker immune resilience, and reduced long-term adaptability. Used correctly, COI is not about alarm—it is about informed planning.
Best practices
- Combine COI estimates with DNA testing and health screening.
- Monitor trends across generations, not only one litter or one mating.
- Avoid repeatedly pairing closely related individuals.
- Use broader population data when available (effective population size, founder representation, and bottlenecks).
Limitations of this calculator
This page provides a practical estimate from relationship categories or a user-supplied r. It does not replace complete pedigree software or genomic-based inbreeding estimates. Real-world outcomes are influenced by many factors, including trait architecture, selective pressure, and existing background inbreeding.
For high-stakes breeding decisions, consult a qualified veterinarian, genetic counselor, or population genetics specialist.
Frequently asked questions
Is this calculator only for animals?
It is intended for educational genetics use and common breeding-program planning contexts. It is not a medical diagnostic tool.
Can two pairings have the same COI but different outcomes?
Yes. COI is a probability summary. Specific outcomes depend on which gene variants are actually present in each line.
What is a “good” COI target?
There is no one-size-fits-all number. Targets vary by species, population size, and conservation goals, but lower long-term accumulation is generally preferred.