Photo of a woman (Professor Susanne Eriksson, SLU) standing with a laptop in her arms in front of a paddock with horses.
Photo: Lisa Chröisty, SLU

Inbreeding in animals – risk, tool or inevitability?

News published:  15/06/2026

Inbreeding between humans is taboo in large parts of the world, but we often view it differently when it comes to animals. While it has helped shape animal breeds, it can also impair health, fertility and performance. How do we avoid crossing the dangerous line to inbreeding depression?

Inbreeding occurs when relatives mate with each other: the more closely related the parents are, the higher the degree of inbreeding and the risks for the offspring. This applies to all species, humans as well as animals.

The risk of inheriting the same harmful genetic variant from both parents then increases – which can lead to reduced fertility, a weaker immune system, poorer growth and poorer performance, says Susanne Eriksson, Professor of Animal Genetics with a specialisation in breeding at the Swedish University of Agricultural Sciences, SLU. 

But the way we view inbreeding varies depending on societal norms and the way we manage breeding. When it comes to animals, we are rarely anywhere near as strict as we are when it comes to ourselves as humans. Why is that? And when do we cross the line before inbreeding leads to inbreeding depression?

Black-and-white photo of two cold-blooded trotters (horses) sniffing each other’s manes while standing in halters in the stable aisle.
Inbreeding: Risk, tool, or inevitability? Photo: Lisa Chröisty, SLU

When inbreeding became a tool 

When we humans began to control animal breeding in such a strict way that we created breeds, inbreeding was used as a tool to obtain groups of animals with certain colours, sizes and other desirable traits. In many cases, there was initially a great deal of genetic variation among the animals, which meant that the breeding often worked well. But the more genetically similar the individuals within the same breed became, the greater the risk has become that things go wrong in the next generations. 

Now that we have created breeds and mainly want to breed within them, some degree of inbreeding is unavoidable; instead, it is a matter of how quickly inbreeding increases, the professor explains. 

Inbreeding depression and popular favourite stallions

During her long career, Susanne Eriksson has worked with several different animal species on issues related to genetics and breeding. Recently, she has focused on several projects concerning inbreeding in horses. 

I think many horse owners know the most popular stallions within the breed, discipline or type of horse they are involved with, and it is not surprising that many want to use the same stallion, but it can be risky, she says. 

The professor believes that the challenge often lies in explaining that inbreeding involves risks both at the individual level – for the unique foal that may be born with defects – and for the entire population. 

The more people use the same popular stallion, the smaller the genetic variation within the breed becomes. This increases the risk of horses being born with genetic defects, but also of a general deterioration in various traits. Less genetic variation also limits opportunities for breeding progress and for adaptations to a changing environment or emerging diseases.

I have great respect for the fact that people often invest a great deal of money, time and heart, and that ultimately it is always the horse owner’s decision. But I can contribute advice and information about what we know from research, she says. 
Photo of a woman standing at her computer in the foreground (Susanne Eriksson, SLU) and horses in a pasture in the background.
Professor Susanne Eriksson works, among other things, with analyses of pedigree data and genetic tools in animal breeding. “My role often becomes translating advanced genetic information into practical advice for animal owners and breeding organisations,” she says. Photo: Lisa Chröisty, SLU

So what can happen if a group of individuals, such as a horse breed, becomes too closely related to one another? 

What may then occur is called inbreeding depression, says Susanne Eriksson. 

Inbreeding depression can be described as a general reduction in traits such as performance, fertility and health due to excessive inbreeding. It is a generally unfavourable effect that can affect horses in many different, negative ways.

When do we cross the line when it comes to inbreeding?

That is probably the most common question I get, says Professor Susanne Eriksson.

Many breeding organisations today recommend keeping the inbreeding coefficient below 6% (roughly equivalent to cousin mating). According to the professor, there are three things that are particularly important when assessing the risks of inbreeding: how much genetic variation there is, how much information we have about relatedness between animals – and how quickly the degree of inbreeding increases.

  • Knowledge: The more information we have about the horses’ pedigree, the more inbreeding we can detect, which often results in a higher estimated degree of inbreeding if we base it on pedigree records. And if you come from a closed studbook, the general degree of inbreeding may already be higher than the rule of thumb suggests.
  • Speed: The most important thing is not whether, but how quickly the degree of inbreeding increases in a population (a horse breed, for example). If the increase is slow, there is time to detect any defects that arise and reconsider – but if it increases too quickly, you can end up in a genetic dead end.
And as always when it comes to probability, there are exceptions: one may be lucky in the genetic lottery, and there are plenty of examples of inbred individuals that are healthy and perform well. That is the reason why some people have positive experiences of inbreeding, and want to continue using it to improve breeding results.

But the professor also emphasises the question of responsibility when it comes to living animals. 

We can make major changes through breeding, and it is therefore important that those changes move in the right direction: combining desirable traits for performance with good durability and health.
Inbreeding should simply be viewed as a risk, but the importance of small increases in the degree of inbreeding should not be exaggerated, she concludes.

Three questions for the expert: Susanne Eriksson

That is fundamentally an ethical question. But if we start from the assumption that we keep animals for utility and enjoyment, and want new individuals to be born, we also need to take responsibility for breeding. In that case, it is better to guide breeding in a desired direction than to leave everything to chance or natural selection – which can also produce undesired outcomes.

Yes, in some cases inbreeding can be used to reinforce desired traits. The problem arises when genetic variation becomes too limited over time, or if an individual is unlucky enough to inherit the same harmful variant from both parents. Then the risks of disease and impaired health increase. Inbreeding can be seen as a risk that must be managed with knowledge and long-term responsibility in order to maintain genetic variation.

For me, sustainable breeding is about balance – combining performance and desirable traits with long-term health, genetic variation and animal welfare.

Portrait photo of a woman (Susanne Eriksson, SLU) taken indoors.
Susanne Eriksson is Professor of Animal Genetics with a specialisation in breeding at SLU. Photo: Lisa Chröisty, SLU

Facts

Susanne Eriksson hold a degree in animal science, and is since 2026 also Professor of Animal Genetics with a specialisation in breeding. She also serves on SLU’s Scientific Council for Animal Welfare, which has recently addressed issues including genetic defects, on behalf of the Swedish Board of Agriculture.

More about her and the research she conducts on an ongoing basis can be found below.

Profile page for Susanne Eriksson


Explanation of terms

Breeding = When humans choose which animals should reproduce, in order to reinforce or reduce certain traits in future generations.

Degree of inbreeding = A measure of how closely related the parent animals are, and therefore how likely it is that the offspring will inherit the same gene variant from both parents.

Genetic variation = Differences in genetic material between individuals within a population. High genetic variation makes populations (for example, a horse breed, or horses of the same breed within a country) more resilient to diseases and environmental changes.

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