COI and Genetic Diversity in the Olde English Bulldogge

The Coefficient of Inbreeding (COI) measures the probability that any two copies of a gene in an individual dog are identical by descent from a common ancestor. A COI of 0% means the parents share no ancestors back to the depth measured. A COI of 25% is what you get when you breed full siblings. The Olde English Bulldogge breed sits in a difficult place: it is reconstructed from a small foundation, so the breed-wide COI floor is higher than most established breeds, which means breeders need to be more deliberate about diversity, not less.

Most OEB breeders never look at COI. They breed the bitch they own to a stud they like and hope for the best. The few who calculate COI ahead of time, balance it against structural and temperament fit, and use the OBBA pedigree archive to find outcrosses are the ones whose lines stay healthy across generations. This is the technical work that separates serious OEB breeders from the rest.

Honest framing. COI is a useful indicator but it is not a verdict. A 6% COI litter from two structurally and temperamentally compatible dogs can produce healthier puppies than a 1% COI litter from two dogs that should never have been paired. Use COI to flag risk, not to decide breedings on its own.

What COI actually measures

COI is calculated from a pedigree by tracing each ancestor on both sides and counting where they appear in common. The deeper the pedigree, the more accurate the calculation. A 5-generation COI is a useful starting point. A 10-generation COI is closer to truth but requires verified ancestry that far back.

0% COI: the parents share no common ancestors in the depth measured. Outcross to the depth available.
1-3% COI: minimal common ancestry. Typical of careful outcrosses.
3-6% COI: moderate. Common in well-maintained working lines.
6-12% COI: linebreeding range. Some shared ancestors at 3 to 5 generations back.
12-25% COI: close linebreeding to direct inbreeding. High risk of expressed recessive issues.
25%+ COI: equivalent to full-sibling or parent-offspring pairing. Avoid.

Pedigree-based COI is what you can calculate from registry data. DNA-tested COI (from companies like Embark or Wisdom Panel) measures actual genetic similarity and is usually higher than pedigree COI because it captures shared ancestry beyond the documented pedigree depth.

The OEB-specific context

The Olde English Bulldogge was reconstructed by David Leavitt starting in 1971 from a small foundation: English Bulldog, American Bulldog, Bullmastiff, and American Pit Bull Terrier in defined ratios. Every modern OEB descends from that original foundation stock, which means there is a built-in genetic floor that no OEB breeder can drop below.

Practical implications:

The breed-wide pedigree COI floor in OBBA-registered dogs sits around 4-6% even for outcrosses, because the foundation is shared.
A 5-gen COI of 8-10% in an OEB is more comparable to a 12-15% COI in a long-established breed.
Crossing to dogs with non-Leavitt foundation lineage (some lines from outside the original Leavitt program) is one of the few ways to drop the breed-wide COI floor.
OEBs with documented cross-registry pedigrees (IOEBA, OEBKC, UKC, LBA) sometimes show genetic diversity that would not be obvious from a single registry's data.

Target COI ranges for OEB litters

For a planned OEB litter:

Under 6.25% (the equivalent of first-cousin pairing or wider): the conservative target. Healthy in most cases.
6.25-10%: acceptable for an experienced breeder linebreeding deliberately to fix a trait. Requires open eyes about which traits are being concentrated.
10-12.5%: the equivalent of half-sibling pairing. Should not be the default and requires strong justification.
Over 12.5%: the equivalent of full-sibling or parent-offspring pairing. Avoid.

The OBBA archive computes COI for every registered dog where pedigree depth permits. Dogs without sufficient pedigree depth show as "COI not calculable" rather than a misleading low number. When planning a litter, look at the COI of the planned puppies, not the parents' individual COIs.

AVK: ancestor loss and effective population size

AVK (Ahnenverlustkoeffizient, Ancestor Loss Coefficient) measures how many of a dog's theoretical ancestors are unique versus duplicated. A 5-generation pedigree has 62 ancestor slots (2+4+8+16+32). If all 62 are different dogs, AVK is 100% (no loss). If 12 of those slots are filled by repeated ancestors, the unique-ancestor count is 50, and AVK is roughly 50/62 = 80.6%.

AVK is a complement to COI. Two litters can have similar COI but different AVK. The litter with higher AVK has more genetic diversity overall, even if some specific ancestors are duplicated. For the OEB, an AVK above 80% in the 5-generation pedigree is healthy. Below 75% is a flag.

How to plan a litter using COI and AVK

The practical workflow for a serious OBBA breeder:

Identify 3 to 5 stud candidates that complement your bitch on structure and temperament. Skip the "COI calculation" for now; that comes second.
For each candidate, calculate the planned-litter COI using the OBBA archive. Most OEB breeders use a hypothetical-litter COI tool or the registry's built-in calculation.
For each candidate, also note the AVK of the planned litter. Higher is better, all else equal.
Identify the shared ancestors that drive any elevated COI. Are these healthy ancestors that have stood the test of time? Or are they ancestors with known issues? Concentrating good genetics is fine; concentrating bad genetics is not.
Pick the candidate that balances COI under 6.25%, AVK above 80%, and structural and temperamental fit. If the structurally best match has a 7% COI and great AVK and healthy concentrated ancestors, that may be the right call. If the lowest-COI option has bad structure, COI alone is not worth it.
Document the decision. Write down why you picked this stud over the others. This documentation is what teaches you across litters and makes you a better breeder over decades.

Using the OBBA archive for COI planning

The OBBA pedigree archive is the largest verified OEB dataset publicly accessible. It contains 16,000+ dogs with 4 to 8 generations of documented pedigree on most of them. For COI planning:

Look up each candidate stud's public profile to see their pedigree, photos, COI, and any titles or test results published
Use the pedigree depth to estimate planned-litter COI. The deeper the documented pedigree on both sides, the more accurate the calculation.
Look at the stud's previous offspring already in the registry. Repeat breedings tell you how the dog passes on traits.
Cross-reference cross-registry papers (UKC, LBA, IOEBA) to extend the pedigree depth where the OBBA record is shorter

A future OBBA tool, the COI calculator, will take two registration numbers and output a planned-litter COI in one click. The engine exists today (the registry runs COI as part of pedigree updates); the public-facing tool is a near-term roadmap item. Until then, manual lookup against the archive is the workflow.

Diversity strategies for working OEB lines

Outcross every 2 to 3 generations. Linebreeding can fix traits but also concentrates risk. A planned outcross every few generations resets the diversity floor.
Use cross-registry studs. Dogs with UKC, IOEBA, or LBA papers in their pedigree often bring diversity that pure-OBBA lines do not have. Verify the cross-registry pedigree before counting on it.
Co-own to keep diversity in the gene pool.A great bitch you cannot keep yourself can be co-owned with a fellow breeder, expanding the breeding pool without giving up the dog's influence on your line.
Import or export. European and Australian OEB programs sometimes offer foundation diversity that North American lines have lost. Cross-border import is feasible.
Track AVK, not just COI. Two breedings with the same COI can have very different AVK. The higher-AVK breeding is usually the healthier litter.