We use Wright’s inbreeding coefficient. The inbreeding coefficient (F) describes the probability that an individual has inherited identical copies of a gene from both parents due to shared ancestors in the pedigree. It is expressed as a percentage (0–100%).
A higher inbreeding coefficient implies an increased risk of inherited weaknesses or diseases. For this reason, the inbreeding coefficient is used as an important decision-support tool in breeding planning.
As a rule of thumb, the inbreeding level should preferably be kept below 5%.
How is the inbreeding coefficient calculated?
Wright’s method is based on the genetic relationship between the parents.
Simply put:
The inbreeding coefficient is half of the genetic relationship between the mare and the stallion.
The calculation is based on shared ancestors and how closely they appear in the pedigree.
Basic principle
For each shared ancestor, a contribution is calculated based on:
- how many generations there are from the mare to the ancestor
- how many generations there are from the stallion to the same ancestor
The contributions from all shared ancestors are summed to produce the total inbreeding coefficient.
Step-by-step calculation
- Identify shared ancestors. Draw or review the pedigree and note which individuals appear on both the mare’s and the stallion’s side.
- Count generations. Count the number of generations from the mare to the shared ancestor and from the stallion to the same ancestor.
- Calculate the contribution per ancestor. Each unique path through a shared ancestor contributes to the inbreeding coefficient.
- Sum all contributions. The total inbreeding coefficient is the sum of the contributions from all shared ancestors.
Examples of common relationships
| Mating | Inbreeding coefficient |
|---|---|
| Parent × offspring | 25% |
| Full siblings | 25% |
| Half siblings | 12.5% |
| Cousins | 6.25% |
Important rules of thumb in breeding
- Goal: Try to keep the inbreeding level below 5%
- Linebreeding: The same individual should preferably not appear closer than the third generation
- Big picture: The inbreeding coefficient is a guide – not an absolute truth
- Data quality: The result is only as reliable as the accuracy of the pedigree
Modern methods
In addition to Wright’s classical method, genomic inbreeding coefficients are now also used. These are based on actual DNA markers rather than pedigree data and can provide a more accurate picture of genetic variation, but they require genetic testing.
Important to know
Pedigree data in Avla is partly user-generated. Missing or incorrect ancestors may affect the result. Always review pedigrees carefully and use the inbreeding calculation as decision support – not as absolute truth.
