What the coefficient of inbreeding means
The coefficient of inbreeding, written F or COI and usually shown as a percentage, was defined by Sewall Wright. It is the probability that the two copies of a gene a pup inherits, one from each parent, are identical because they come from the same ancestor.
A COI of 0% means the parents share no known ancestor. The higher the number, the more of the pup’s genes are expected to be the same on both sides.
Typical values for related pairs
These are the standard textbook values for a pup whose parents are related in one single way:
- Parent mated to offspring: 25%
- Full siblings: 25%
- Half siblings: 12.5%
- Grandparent mated to grandchild: 12.5%
- First cousins: 6.25%
Why breeders watch it
Inbreeding raises the chance that a harmful recessive gene is inherited from both sides, which can show up as smaller litters, weaker pups or health problems. Some breeders also use controlled line breeding on purpose to fix a trait, so a higher number is a trade-off to judge, not an automatic no.
A COI is a planning tool. It does not replace the advice of an experienced breeder, your breed club or a vet.
What a pedigree-based COI cannot see
The calculation only knows the ancestors you have recorded. An ancestor entered as unknown is treated as unrelated to everyone, so the true figure can be higher than the one shown.
A low COI from a pedigree that goes back only two generations is therefore weaker evidence than the same number from five or six generations.
Roan and Dalmatian pairings
Beyond relatedness, some colour pairings carry their own risk. Pairing two Roan or two Dalmatian animals can produce lethal white pups, and Pigree flags those pairings as well.
Checking a pairing before the mating
Record the boar and the sow with as many known ancestors as you can, pick them as the parents in a new litter, and read the result before you commit. If it is higher than you like, compare it with another candidate.
- Enter the parents and their parents, so the calculation has ancestors to work with.
- Create a litter and choose the boar and the sow.
- Read the severity level and the list of shared ancestors.
- Try another pairing if the result is too high.
Frequently asked questions
What is a safe COI for guinea pigs?
There is no single safe number. Lower is generally safer, and what is acceptable depends on the goal and on how deep the pedigree is. Pigree shows a severity level and suggests pairs below 5%, but ask an experienced breeder or a vet about your own animals.
Can I calculate COI without a full pedigree?
Yes, but the result only covers the ancestors you know. Unknown ancestors count as unrelated, so the real value may be higher.
Does Pigree calculate the inbreeding coefficient for free?
Yes. The inbreeding warning is part of the Free plan.