When breeders study a pedigree, familiar names begin to appear again and again. Sometimes the repetition is easy to spot: an influential stallion appears on both sides of a pedigree, perhaps only a few generations back. In other cases, two pedigrees look quite different, even though many of their ancestors are already widely represented throughout the breeding population.
This is where two useful terms come in: inbreeding and relationship.
They are connected, but they do not describe the same thing. Inbreeding helps us understand the expected effect of shared ancestry in a particular mating. A relationship percentage, as used by the KWPN, places a horse or possible foal in relation to a wider breeding population. Read together, the figures can help breeders look beyond the names on a page and consider what a mating may mean for the generations that follow.
A proposed mating raises two separate questions:
How much common ancestry do the mare and stallion share, and what inbreeding coefficient is expected for their foal?
How closely connected is the possible foal to the relevant breeding population?
The first concerns a specific pairing. The second concerns the foal’s place in a population.
In its breeding tool, the KWPN shows both an inbreeding percentage and a relationship percentage for a potential foal. The inbreeding figure helps breeders assess how closely related the mare and stallion are through their pedigrees. The relationship percentage indicates how the possible foal relates to the selected breeding direction and its population. A lower relationship percentage means the foal is less closely connected to that reference population; it does not, on its own, tell us whether the foal will be a better sport horse.
| Measure | What it asks | What it helps breeders consider |
|---|---|---|
| Inbreeding coefficient of the foal | How much shared ancestry is expected in this specific mating? | Whether the mare and stallion’s common ancestors bring too much related blood together |
| Relationship percentage | How closely connected is the horse or possible foal to a defined population? | Whether its blood is widely represented or may contribute to blood distribution |
| Relationship between the parents | How related are the mare and stallion to each other? | Why the foal may have a higher or lower inbreeding coefficient |
The term relationship is not used in exactly the same way in every genetic database. It can refer to the relatedness of two individuals, or to the relationship between an animal and a reference population. So it is important to check what a studbook means by its particular percentage.
A horse receives genetic material from both its sire and dam. When the parents share ancestors, the foal may inherit copies of the same ancestral allele through both sides of its pedigree. The inbreeding coefficient estimates the probability that, at a given gene location, the foal’s two alleles are identical by descent—meaning they came from the same ancestor.
It is a pedigree-based expectation, not a prediction that a particular disease or trait will occur. It also does not measure a horse’s overall quality, athletic potential or future soundness.
Classical pedigree examples illustrate how the parents’ relationship can affect the foal’s expected inbreeding coefficient:
| Relationship between the parents | Expected inbreeding coefficient of the foal |
|---|---|
| First cousins | 6.25% |
| Half-siblings | 12.5% |
| Full siblings | 25% |
| Parent and offspring | 25% |
These examples assume no additional shared ancestry that would increase the result. In real pedigrees, the calculation can include multiple common ancestors and ancestors that are themselves inbred. A repeated name is therefore a reason to look more closely, but the number and position of all shared ancestors matter. A five-generation pedigree displayed on a page may not show every ancestor included in a database’s calculation.
The KWPN relationship percentage answers a population question. It indicates how closely a horse is related to the relevant breeding direction and its reference population. The KWPN describes its reference group as the younger generations in the population—foals through two-year-olds—and says the percentages are recalculated against the most current population. Relationship classes and percentages differ by breeding direction, so figures for dressage, jumping, Gelder and harness horses should not be compared as though they use one common scale.
This also explains why a horse’s relationship percentage can change even though its own pedigree stays the same. If a stallion is used extensively, more of his sons and daughters enter the population. The population around him changes, and so can his relationship to it.
A lower relationship percentage can make a horse or potential foal relevant to blood distribution. It may represent ancestry that is less widely present in the reference group. But the percentage is not a complete DNA profile, and it does not prove that a horse is genetically distant from every potential mate. It is a population indicator whose meaning depends on the reference group and the calculation method.
A mare and stallion can be relatively unrelated to one another and produce a foal with a low inbreeding coefficient, while both still descend from bloodlines that are common in the wider population. Their foal may not be highly inbred as an individual, yet the pairing may add little new blood to the population.
The opposite can also happen. A stallion may carry blood that is less common in the population, but if the mare shares several of his ancestors, their foal may still have a higher inbreeding coefficient.
That is why neither percentage can replace the other. We need to ask both whether the mare and stallion are suitable for one another and how their blood fits into the population around them.
This matters in sport-horse breeding, where successful bloodlines can spread internationally and quickly. Individual matings may remain within a desired inbreeding guideline, while the same influential ancestors become increasingly common across the population. Over generations, that can make future stallion choices more restricted.
Inbreeding figures can also have practical consequences in formal stallion selection. At the 2024 KWPN Stallion Show, catalogue number 457, a Las Vegas × Bordeaux stallion, was not selected for the performance test. Contemporary reporting stated that inbreeding was one reason he was left out.
This is a useful example of pedigree and inbreeding being considered in a real breeding decision, rather than remaining abstract figures in a database. The report does not tell us the exact calculation behind the decision or everything the committee weighed. It does not mean the stallion was unhealthy or lacked quality. It does show that genetic considerations can influence which horses are given the opportunity to continue in a breeding selection process.
For breeders, this reinforces the value of looking beyond appearance and performance potential alone. A horse may have qualities worth preserving, while the way its blood fits into the population still deserves careful thought.
Breeders sometimes use the term linebreeding when they deliberately repeat a valued ancestor to preserve or strengthen characteristics associated with that horse or family. The intention may be different, but genetically the mating still brings related blood together. The KWPN describes linebreeding as a form of inbreeding and cautions that repeated ancestors may concentrate undesirable traits as well as desired ones.
That does not make every repeated ancestor a problem. Nor does it mean that an influential horse should disappear from a pedigree simply because it occurs more than once. It does mean the reasoning should go beyond, “This was an exceptional horse, so seeing him twice must be positive.”
We need to consider where the ancestor appears, how often, through which branches and alongside which other shared ancestors. Repetition may increase the chance of concentrating particular genetic contributions, but it does not guarantee that a valued trait will be fixed. Many characteristics important to sport performance are influenced by multiple genes as well as development and environment.
The name given to the breeding strategy does not change the underlying pedigree relationship.
Successful stallions become popular because breeders see qualities they want to use. Their influence can bring real progress. The long-term challenge comes when a small number of sires contribute a large share of the next generation.
As their sons and daughters enter breeding, the same ancestors may begin to appear on both sides of more pedigrees. A generation later, breeders can find that stallions who initially seemed different already carry much of the same influential blood as their mares.
A 2024 study modelling breeding strategies in the Friesian horse population identified unequal sire contributions as an important factor in the rate of inbreeding. In the study’s simulations, selection based on a stallion’s mean kinship to the population could help reduce that rate. The results are specific to the Friesian population, but they illustrate why individual mating decisions and population-level blood distribution are connected.
This does not mean a widely used stallion should automatically be avoided. It means his use should be considered in the context of the mare, the pairing and the population. Sometimes the best match for a mare may come from a widely represented line. In another case, a suitable stallion with less common blood may keep more options open for the future.
As inbreeding increases homozygosity, it can increase the chance that a foal receives two copies of the same harmful recessive variant from a shared ancestor. Inbreeding can also contribute to inbreeding depression: a reduction in traits such as fertility or survival associated with increasing inbreeding. These are population and probability concerns, not a certainty about the health of any one foal.
Research in horses is specific to the breed and traits studied. For example, a study of 22,799 Pura Raza Española mares reported associations between inbreeding and several reproductive traits. It is useful evidence for why reproductive fitness is part of genetic management, but its results should not be treated as a direct prediction for every breed or individual horse.
A low inbreeding coefficient does not guarantee that a foal will be healthy, sound or durable. Nor does a higher coefficient mean that a particular horse will inevitably have health problems. The figures help breeders assess risk and ancestry; they do not replace veterinary information, health testing, knowledge of the family or observation of the horses themselves.
Pedigree calculations are practical tools, but their accuracy depends on how complete and correct the recorded ancestry is. Missing ancestors or limited historical records can affect a result. IFCE notes that shared ancestors beyond the visible five generations can contribute to a horse’s inbreeding coefficient, so a calculation based only on the displayed pedigree may underestimate it.
Genomic methods can add another perspective. One method looks for runs of homozygosity (ROH), long stretches of matching DNA within an individual genome. This can help estimate realized genomic inbreeding, which may differ from the expectation calculated from a pedigree. The two approaches are related, but they do not answer exactly the same question.
For most mating decisions, pedigree analysis remains a valuable starting point. Where genomic information is available and relevant, it can add evidence, but it does not remove the need to consider the individual horses, their families and the purpose of the mating.
At Maverick Stables, we want to breed sport horses with the potential to be functional and durable, and we hope to contribute to healthy horses with responsible levels of inbreeding. We also want to keep our breeding options open rather than narrowing our programme around a small number of heavily used bloodlines.
For us, the KWPN’s recommendation to remain below 5% is an upper limit, not a target. We do not want to plan matings above 5%. Where a suitable combination allows, we prefer to keep the projected inbreeding coefficient below 3%.
These are our own breeding preferences, not a universal boundary between a “safe” and an “unsafe” mating. The percentage is one part of the decision, alongside the mare, the stallion, their health and performance, the dam line and the qualities we want to preserve or improve. A lower figure does not guarantee a healthy or durable horse, but keeping inbreeding low where possible helps us avoid unnecessarily concentrating related blood and preserve options for future generations. The KWPN’s current tool advises breeders to remain below 5%.
We start with the mare: her conformation, movement or jumping ability, health, temperament, performance and the qualities we see in her dam line. Then we consider what a stallion may add, where the pedigrees overlap and what the inbreeding and relationship figures tell us about the proposed foal and the wider population.
If several stallions could suit the mare, the relationship percentage may help us understand whether one option represents blood that is less widely represented. But a low relationship percentage does not make an unsuitable stallion the right match. Equally, a widely represented line is not automatically unsuitable if there is a sound reason for the combination and the rest of the mating has been considered carefully.
For us, genetic diversity supports the breeding decision; it does not replace it.
The value of understanding inbreeding and relationship becomes especially clear when we think beyond the foal expected next year.
If a filly later becomes a broodmare, what choices may be available to her? Will her pedigree leave room to select for health, performance and type without repeatedly bringing the same blood together? Or are we creating a combination today that could make those future choices more difficult?
We cannot predict which stallions will be available in ten or twenty years, and breeding is not a mathematical exercise with a guaranteed outcome. But we can avoid unnecessarily closing doors. When a mare already carries an influential line strongly, repeating it should be a deliberate decision. If another suitable stallion offers less widely represented blood, that may be an additional benefit for the generations ahead.
That is why we consider the mare, the proposed foal and the population around them. Breeding is not only about producing the next horse. Each mating becomes part of the pedigree and helps shape what remains possible for the generation after it.
The mare comes first. The pedigree gives us context. The numbers help us think beyond the next foal.
Inbreeding estimates the shared ancestry brought together in an individual foal from a particular mating. In the KWPN context, the relationship percentage shows how closely the horse or possible foal is connected to a reference population. The two figures answer different questions.
We do not want to plan matings above 5%. Where a suitable stallion is available, our preference is to keep the projected inbreeding coefficient below 3%. This is our own breeding aim; it does not replace assessment of the complete pedigree, the individual horses or the purpose of the mating.
The KWPN advises breeders using its mating tool to keep the expected inbreeding percentage below 5%. Maverick Stables treats 5% as an upper limit and aims lower where a suitable combination makes that possible.
Linebreeding describes the intention to repeat selected ancestors or families. Genetically, it still involves related blood, so the shared ancestry and resulting inbreeding should be assessed carefully.
Not by itself. A low relationship percentage indicates that the blood is less closely represented in the reference population. It does not tell us whether the stallion complements the mare in health, conformation, temperament, movement or performance.
Yes. Sport-horse breeding is international, and influential bloodlines are used across studbooks. A different studbook or country does not automatically mean an outcross; the actual pedigrees need to be compared.
No. It is one piece of information about shared ancestry. It cannot guarantee health or durability, which also depend on the horses themselves, hereditary conditions, family history and many other factors.
This article is intended for general educational purposes. Percentages depend on the data, reference population and calculation method used. For a specific mating, consult the relevant studbook or a qualified breeding adviser and consider the complete pedigrees and individual horses.
