Genetics
Inbreeding Coefficient (COI) in Dogs: What It Is and Why It Matters
The coefficient of inbreeding (COI) is the single most important number in a dog's pedigree that most buyers never see. Here's what it means and why it matters for health.
August 20, 2024 · 6 min read
What Is COI?
The Coefficient of Inbreeding (COI) is a mathematical measure of the probability that an individual has inherited identical copies of a gene from a common ancestor. A COI of 0% means the parents share no known common ancestors — maximum genetic diversity within the known pedigree. A COI of 25% means the parents are as related as a full sibling pair. In purebred dogs with small founder populations, COI can be surprisingly high even in non-intentionally inbred pairings.
COI Thresholds: What the Research Says
- 0–5%: Low inbreeding. Preferred range for maximum genetic health.
- 6.25%: Equivalent to a first-cousin breeding.
- 12.5%: Equivalent to a half-sibling breeding. Noticeable increase in genetic health risks.
- 25%: Equivalent to full sibling or parent-offspring breeding. Significant health implications.
- Most healthy purebred breeds average 5–15% COI at the population level.
How Genomic COI Is Calculated
Unlike pedigree-based COI calculations (which only go back as far as the recorded pedigree), genomic COI is calculated from actual DNA — comparing thousands of genetic markers across the genome to identify shared homozygous regions. Genomic COI is more accurate than pedigree COI because it captures inbreeding from generations beyond the recorded pedigree.
Why COI Matters for Health
High COI increases the proportion of the genome where both copies of a gene are identical (homozygosity). This increases the likelihood that recessive disease alleles — normally masked by a healthy dominant copy — will pair with themselves, causing disease. It also reduces immune system diversity (MHC diversity) and can compromise adaptability. Responsible breeders aim to select pairings that minimize COI while maintaining breed standards.