Inbreeding Coefficient Calculator
Measuring Shared Ancestry in a Pedigree
The inbreeding coefficient, F, is the probability that an individual inherited two identical copies of the same ancestral allele — one through each parent — because both parents descend from a common ancestor. It's a core tool in pedigree analysis for livestock breeding, conservation genetics, and population biology, quantifying how much shared ancestry sits behind a given mating rather than describing any specific trait.
The Formula
n1 and n2 are the number of generational links from each parent back to their shared common ancestor, and F_A is that common ancestor's own inbreeding coefficient (0 if the ancestor is assumed unrelated to itself through other lines). This is Sewall Wright's path-counting method applied to a single shared-ancestor path.
How Path Length Affects F
Holding F_A at 0, the coefficient falls by half with every additional generational link between the parents and their common ancestor:
| n1 + n2 | Exponent (n1+n2+1) | F (single path) |
|---|---|---|
| 1 | 2 | 0.25 |
| 2 | 3 | 0.125 |
| 3 | 4 | 0.0625 |
| 4 | 5 | 0.03125 |
| 5 | 6 | 0.015625 |
| 6 | 7 | 0.0078125 |
Where This Calculation Is Used
- Livestock and companion animal breeding — breed registries track inbreeding coefficients to avoid compounding recessive genetic disorders in closed gene pools.
- Conservation and captive breeding programs — managing small, isolated populations to keep average inbreeding coefficients low and preserve genetic diversity.
- Population and quantitative genetics coursework — the standard worked example for Wright's path-counting method.
How to Use This Calculator
- Enter n1, the number of generational links from Parent 1 to the shared common ancestor.
- Enter n2, the same for Parent 2.
- Optionally enter the common ancestor's own inbreeding coefficient, F_A (defaults to 0).
- Select Calculate to get F for that single shared-ancestor path.
Related Calculations
For the general coefficient of relatedness between two individuals, see the Genetic Relatedness Calculator, or explore allele-level population math with the Allele Frequency Calculator.