"Understanding the Coefficient of Inbreeding (COI) in Poultry"
If you breed chickens long enough, you’ll hear about inbreeding depression, falling fertility, weaker chicks, poor hatch rates. The number breeders use to keep it in check is the coefficient of inbreeding, or COI. It’s less intimidating than it sounds.
What COI actually measures
COI is the probability that a bird’s two copies of a gene are identical by descent , that is, both copies came from the same ancestor somewhere in the pedigree. Developed by geneticist Sewall Wright, it’s expressed as a percentage from 0% to 100%.
- 0% means no common ancestors appear in the pedigree you’ve recorded, an outcross.
- A higher COI means the same ancestors appear on both the sire’s and the dam’s side, concentrating their genes, good and bad alike.
A bird’s COI equals the kinship between its two parents. So COI is really a statement about a pairing.
The numbers you’ll actually see
Some common matings and the COI of their offspring:
| Pairing | Offspring COI |
|---|---|
| Unrelated birds | 0% |
| First cousins | 6.25% |
| Half siblings | 12.5% |
| Full siblings | 25% |
| Parent × offspring | 25% |
A rough working guide for poultry:
- Under ~6.25%, low. Comparable to a cousin cross or less; generally safe.
- 6.25%, 12.5%, moderate. Typical of deliberate line breeding; fine if you track it.
- Above ~12.5%, high. Vigor and fertility risk rises; breed only with a clear goal.
These aren’t hard cliffs, they’re guideposts. Plenty of strong lines were built with periods of close breeding followed by an outcross to restore vigor.
Why inbreeding isn’t always bad
Line breeding, mild, controlled inbreeding toward a common excellent ancestor, is how breeders fix good traits so a line breeds true. The art is doing it on purpose and knowing when to stop. Watch for the warning signs of going too far:
- Smaller clutch and lower fertility
- Reduced hatchability
- Slower-growing, less vigorous chicks
- More deformities
When you see these, it’s time to outcross, bring in unrelated blood, and reset.
The depth problem
Here’s the catch: COI is meaningless if your pedigree is shallow. Two birds can look unrelated at the parent level but share a grandsire. COI only reveals that if your records go back far enough. The more generations you track, the more honest your COI.
Check a mating before you make it
The most useful move is to calculate a pairing’s COI before you set eggs. That way you choose the cross deliberately instead of discovering a problem at hatch.
ChickenAncestry does this automatically. Record each bird’s sire and dam, and the app computes every bird’s COI for you, plus a test-mating tool: pick a cock and a hen and instantly see the COI their chicks would have. No spreadsheets, no manual pedigree math.
Want the bigger picture first? Read getting started with breeding or learn how clan mating keeps COI low in a closed flock. Then try the test-mating tool free →.
ChickenAncestry tracks every bird's sire and dam, draws the pedigree, and computes inbreeding (COI), free to start.
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