Pedigree inbreeding is an estimate of the probability that an animal has inherited identical copies of a gene from a common ancestor through both its sire and dam, based on recorded pedigree relationships. The pedigree inbreeding percentage displayed on an animal profile helps breeders identify how inbred the animal is based on pedigree information, enabling them to make informed breeding decisions and manage inbreeding levels within their herd.
The Australian Wagyu Association (AWA) maintains the most complete pedigree records available for the Wagyu breed, tracing ancestry back to the foundation animals imported from Japan during the 1990s. However, these pedigrees typically truncate 2–4 generations prior to export. It is well understood that considerable inbreeding existed within the Japanese Black population before exportation. As a result, pedigree-based inbreeding estimates cannot account for this missing ancestry information. Based on the available records, the average pedigree inbreeding level of the imported foundation animals was approximately 6%.
How Pedigree Inbreeding is Calculated
Pedigree inbreeding is calculated by tracing common ancestors that appear on both the sire’s and dam’s side of a pedigree. The closer the common ancestor is to the individual, and the more frequently that ancestor appears within the pedigree, the greater the expected level of inbreeding. The calculation assumes that each parent contributes 50% of its DNA to its offspring and that this contribution is halved with each successive generation.
The expected proportion of shared DNA decreases by half with each generation of separation, as shown below.
| Relationship Category | Examples | Steps Apart | Average DNA Shared |
| First-degree | Parents, Offspring, Full Siblings | 1 step | 50% Relatedness |
| Second-degree | Grandparents, Grand progeny, Aunts/ uncles, half siblings | 2 steps | 25% Relatedness |
| Third-degree | First Cousins, Great Grandparents, Great Grand progeny | 3 steps | 12.5% Relatedness |
| Fourth degree | First cousins once removed, Great Aunts/Uncles | 4 steps | 6.25% Relatedness |
The table above describes the expected relatedness between two animals. These relationships are then used to calculate an individual’s inbreeding coefficient, which estimates the probability that identical genetic material has been inherited from a common ancestor.
Understanding Pedigree Inbreeding
To understand pedigree inbreeding, as general rules:
- The closer the relationship between the sire and dam, the higher the expected inbreeding level of their offspring.
- The more common ancestors shared between the sire and dam, the higher the expected inbreeding level.
- Repeated use of the same influential animals throughout a pedigree can increase inbreeding, even when the relationship is several generations removed.
Limitations
Pedigree inbreeding has some limitations because it is based on expected inheritance patterns rather than the DNA actually inherited by an individual animal.
While each offspring receives 50% of its DNA from each parent, the specific segments of DNA inherited are determined randomly. As a result, full siblings can differ substantially in both the amount of DNA they share and the amount of inbreeding they inherit.
Pedigree inbreeding assumes average expected relationships and therefore cannot account for these differences. Two animals with identical pedigree inbreeding percentages may have inherited very different amounts of inbred DNA, which can influence performance and trait expression.
However, in the absence of other data, pedigree inbreeding values can be used to effectively manage inbreeding levels, particularly when pedigree depths is available.
Why Inbreeding Matters in Wagyu
Inbreeding increases the likelihood that an animal inherits identical copies of DNA from common ancestors, reducing genetic diversity within the population.
Animals with high levels of inbreeding may experience reduced:
- Developmental growth
- Fertility
- Survival
Animals also carry recessive genetic variants that may not affect performance when only one copy is present. As inbreeding increases, there is a greater chance an animal inherits identical copies of these variants from a common ancestor, increasing the likelihood that harmful genetic defects are expressed.
While linebreeding can be used to concentrate desirable genetics within a herd, excessive use of closely related animals can increase inbreeding levels over time. Introducing less-related or outcross animals can help maintain genetic diversity and reduce the accumulation of inbreeding.
Therefore, managing inbreeding within the herd is vital for overall herd performance and welfare.
Key takeaway
Pedigree inbreeding uses recorded ancestry to estimate the likelihood that an animal inherited identical copies of DNA from a common ancestor. As pedigree inbreeding increases, genetic diversity is expected to decrease. While it is a valuable tool for managing mating decisions, it should be viewed as an estimate based on pedigree records rather than a measurement of the actual DNA inherited by an animal.
