Effective population sizes

Dog running to the sea
Megan Sennett © / The Kennel Club

Recognising the importance of managing the rate of inbreeding, The Kennel Club's population analysis reports allow breeders to review the unique situation for each breed. 

Why is this research needed?

Pedigree dogs have many advantages because we know their ancestry and can predict the way that they will turn out. This helps us to know:

  • how big they will grow
  • their exercise needs
  • what breed-related health problems they are at risk of, helping breeders to know which DNA tests to give the parents before they are bred from, none of which is available for dogs of mixed ancestry

It also means that they tend to have a more closed gene pool and so we have to manage the rate of inbreeding at sustainable levels to ensure genetic diversity is preserved, as the lower the genetic diversity, the greater the risk that certain health conditions will begin to surface.

Topics that relate to our research

Understanding the effective population of a breed

Effective populations of more than 100

  • Effective population sizes above 100 are sustainable
  • The rate of inbreeding has remained relatively steady and is within the level thought to be sustainable. This means that there is a suitable balance between selective breeding and inbreeding, therefore the genetic diversity is being effectively managed.

Effective populations of between 50 and 100

  • The rate of loss of genetic diversity within a breed or population increases dramatically when the effective population size is <100
  • This may imply that the genetic diversity of the breed is steadily being lost from the population, or that the rate of inbreeding was high in previous years but is gradually slowing down.

Effective population of less than 50

  • An effective population size that is less than 50 is considered to be at high risk of detrimental effects of inbreeding.
  • The breed is at risk of detrimental effects of inbreeding which could increase the chances of the breed being at risk for both known and unknown inherited disorders. The population is also at risk of inbreeding depression, which is an overall decrease in general fitness, or general health, and may reduce litter sizes and fertility across the breed.
Inbreeding and selection - the balancing act

How do we define Inbreeding?

Inbreeding is the mating of related individuals, and the sharing of genetic material from common ancestor(s).

How do we define selection?

Selection is the deliberate breeding of dogs that have been chosen to produce offspring based on particular desired characteristics and genetic traits.

Inbreeding vs selection

You might not be surprised to hear, that dogs that are related tend to look like each other.

What are the effects of inbreeding?

  • Consistent inbreeding increases the rate of inbreeding in the whole breed
  • Inbreeding can reduce genetic diversity of the breed
  • Decrease in effective population sizes
  • Impacts of inbreeding include a reduction in litter size and fertility, and also health in general.
Popular sires - the biggest contributor to inbreeding

What are popular sires?

Popular sires are male dogs that are used to produce large numbers of puppies. These dogs are often chosen because they have characteristics that are desirable to a breeder. Popular sires are often used to help improve the breed, but excessive use of any males can
impact the health of the population.

Goal for breed management

To manage genetic diversity within the breed to maximise the variety of healthy and desirable dogs within the gene pool. It is not necessary to use all dogs in the population (especially those with undesirable traits). Health, genetic diversity and good husbandry must work together in balance.

What are the effects of a popular sire?

  • The over use of a sire can shrink the size of a breeds gene pool. The gene pool is the total amount of genetic variation within a breed. Unless new dogs are introduced into a breed, the gene pool will become smaller over time – popular sires speed up this process.
  • Every litter produced by a popular sire means one less opportunity for another male to reproduce. The genes from the other dogs which were not chosen for mating, will become rarer and may even disappear from the gene pool entirely.
  • Extreme degrees of inbreeding can lead to inbreeding depression (reduced litter size, increased puppy mortality, reduced fertility, a shorter lifespan etc.) and an increased risk of developing both known and unknown inherited disorders.
  • The smaller the gene pool becomes, the more difficult it can be to find unrelated individuals for mating. Smaller gene pools may be more difficult to manage and may result in increasing levels of inbreeding.

Aim of the study

The study’s aim was to analyse population data for all 215 breeds currently recognised by the UK Kennel Club between 1980 and 2014 to determine trends in registration, rate of loss of genetic diversity, degree of inbreeding and popular sire usage.

Data and method

All electronic pedigree information held by the Kennel Club was used to calculate inbreeding coefficients for every registered individual born from 1980 to 2014. The rate of change of average inbreeding coefficient per breed (rate of inbreeding) describes the rate of loss of genetic diversity over this period.

The findings

  • Across all breeds the rate of inbreeding appeared highest in the 1980’s and 1990’s. The degree of inbreeding tended to decline after 2000.
  • This trend occurred in both popular and rarer breeds, although it was more pronounced in rarer breeds.
  • Rates of inbreeding appears not to correlate with population size.
  • The use of popular sires was apparent in all breeds and is likely to be the biggest contribution to inbreeding.


A large decline in breed genetic diversity occurred in the 1980’s and 1990’s. However, since the turn of the century, the general trend is that the rate of inbreeding has slowed drastically, implying that many breeds may be seeing some replenishment of genetic diversity.

Goal for breed management

To manage genetic diversity within the breed to maximise the variety of healthy and desirable dogs within the gene pool. It is not necessary to use all dogs in the population (especially those with undesirable traits). Health, genetic diversity and good husbandry must work together in balance.

Where can I read more about the research and its findings?

To find a detailed summary of the findings please read Trends in genetic diversity for all UK registered pedigree dog breeds, published in the Journal of Canine Genetics and Epidemiology.

Results for individual breeds

If you would like to find the results for a particular breed, please email Health (The Kennel Club) .

Further reading

If unchecked, inbreeding levels can rise in a breed, and although its effects may not initially be noticeable, this increase can have a significant impact on the health and welfare of future generations.

Read more:

Questions and answers

Is an inbred dog guaranteed to be unhealthy?

Any animal that is selectively bred for predictable characteristics (such as temperament, health and size) has a closed population and so will have a higher rate of inbreeding than those that are not. This does not need to be to the detriment of good health.

Using the Inbreeding Co-efficient (CoI) calculators enables breeders to minimise the degree of inbreeding in future litters and reduce the risk of puppies developing inherited health problems. It is important to note that the inbreeding co-efficient is a measure of risk, rather than a direct measure of health. It is possible that two closely related dogs do not have the same autosomal-recessive genes, while two seemingly unrelated dogs do - it’s all down to chance.

Although the COI is not a guarantee of health, it is a measure of risk with a higher CoI suggesting a higher risk. If a breeder DNA tests their dogs, they are taking steps to avoid a known risk. By using CoI calculators when selecting potential mates, they are reducing the risk of unknown conditions.

If inbreeding causes so many problems, why don’t we just use crossbreeds?

Unfortunately crossbreeds are susceptible to the same health problems as pedigree dogs. As an example, a ‘Labradoodle’ is the result of a cross between a Labrador and a Poodle. Both of these breeds can be susceptible to hip problems and simply crossing them will only exacerbate the problem and not eradicate it.

When breeding from a pedigree dog there is a predictability of specific breed traits, meaning that people can get the right dog for their lifestyle and give a dog a home for life. These predictable traits, including behaviour and temperament, care needs, and their health predisposition, enable breeders to know which DNA tests to give the parents so that they can avoid the problems in the next generation, and this is currently not the case with crossbreeds.

Furthermore, crossbreeds are susceptible to the same reduction in genetic diversity the more generations are bred from.

There are many breeds that have estimated effective population sizes above the recommended level of 100, but there are also a lot of breeds below this. Is The Kennel Club concerned about this?

The Kennel Club wanted to find out the estimated effective population size (EEPS) for every breed, as this tells us the size of the gene pool. It should ideally be above 100, but we have also found out that the rate of inbreeding is slowing down in many breeds, meaning that the estimated effective population size can and will improve.

This has happened for many reasons, one of which being that we have been able to embrace the advances in science to develop our Inbreeding Co-efficient calculator, which enables people to look at the inbreeding coefficient of any particular mating to decide if it is a good idea. This information, and the latest report we have conducted for every single breed in the UK, enables breeders to make the best choices for their dogs and their breed. The rate of inbreeding can also slow down or improve due to other reasons, such as when dogs are brought in from overseas.

We also need to remember that inbreeding co-efficients are a measure of risk, they do not necessarily mean there will be a problem in any given puppy or with any given breed. Many of the breeds with high EEPS do not have an unduly high burden of a single gene disease.

The main thing that we need to ensure is that the breeds are healthy and whilst reducing the rate of inbreeding is an important part of this, different factors need to be considered in every mating for the good of the pups and the breed overall.

What we are doing is supplying breeders with lots of information so they can carefully calculate which dogs are the best match for one another and so long as this information is used, we can ensure breeds and individual pups continue to be healthy.

Popular sires are identified in the research as one of the contributors to the loss in genetic diversity. Can The Kennel Club not set a limit on the numbers of puppies/litters a sire can have?

Unfortunately due to the vastly different population sizes, it is impossible to set a threshold on the number of puppies from a single sire across all breeds. An example of this could be - a Labrador male siring four litters in a year would be siring about 0.1% of all puppies registered that year. Compare that to an Otterhound dog doing the same and the dog would sire virtually all registered puppies that year.

Furthermore, we need to remember that popular sires are popular normally because they have good characteristics in terms of health and temperament. There is nothing wrong with a popular sire producing pups if those pups don’t themselves breed, as they will be making zero genetic contribution to future generations - e.g. some guide dog sires might produce good progeny and so may be selected for more frequent use, but the pups are unlikely to ever be bred from - and it means the pups get the best health and temperament possible. It is not the number of progeny, but the number of breeding progeny that is most important and we do not know which offspring will be used for breeding at the point of registration.

If a limit is set at a sire producing 100 puppies, but then 10 of the sire’s sons produce another 100 each, this is as much of a problem genetically as a sire having 500 progeny.

Directly monitoring contributions is a more effective way of identifying ‘popular sires’ and The Kennel Club has recently been conducting research in this area so that we can look more closely at genetic contributions. Furthermore, we are working on a ‘popular sire’ tool to add to our Inbreeding Co-efficient calculator, which will enable those with breeding bitches to see how many times a sire has been used.

What is The Kennel Club planning to do about the problem of popular sires?

The Kennel Club advises against overuse, particularly of males, for breeding and publishes the number of puppies each dam/sire has produced. 

We are conducting research into directly monitoring the genetic contributions made by individuals to subsequent generations and how best to convey this information to breeders via our Inbreeding Co-efficient calculators.

The Kennel Club says that one answer to improving the rate of loss of genetic diversity is through outcrossing. Has this been done before?

Dogs that have been outcrossed have been registered by The Kennel Club for a long time. Examples include crossing the Irish Red and White and the Irish Setter, inter-variety Belgian Shepherd Dog matings, the Bull Terrier to the Miniature Bull Terrier and the registration of progeny from a Dalmatian/Pointer mating, for a low uric acid (LUA) gene.

Why not make outcrossing mandatory for many more breeds?

Outcrossing into the gene pool can have a positive effect but it is not a silver bullet. When you bring a dog in in this way you need to remember that the dog will then be a common ancestor to all dogs with that version of the gene, which will be desirable. So whilst an outcross is a useful tool, it doesn’t mean you can stop managing inbreeding.

Furthermore, if you get breeders who favour the dog with the outcrossed characteristics and others who favour those without it, you could split the breed in two, reducing genetic variation still further. So we need to work carefully with breeders to ensure our actions are not counterproductive.

What process does a breed need to go through to decide if an outcross will be allowed?

If a breed is considering crossing with a different breed, the first step would be to begin discussing this with the health team at The Kennel Club. Any breeder considering an outcross programme will need bespoke advice from geneticists to help formulate any breeding strategies. Many breeds concerned about genetic diversity and inbreeding would benefit from simpler strategies, such as reducing use of popular sires, using a wider number of males and females from different sub-populations within the breed, and careful use of international animals. Breed-specific advice can be sought from our health team.

Email The Kennel Club health team


Why don’t you set a limit on the co-efficient of inbreeding (COI) of dogs that you will register?

It is impossible to set a sensible and consistent threshold for CoI due to varying levels of genetic diversity within breeds and differing amounts of pedigree information between dogs. For some breeds, an abundance of pedigree information (enabling detection of distant ancestral relationships) and depleted genetic variation may mean it would be very difficult or impossible to meet a particular threshold.

Furthermore, while the CoI describes the risk of the detrimental effects of inbreeding occurring in an individual, simply seeking to minimise individual CoIs is not an effective way to manage the rate of loss of genetic diversity within a breed. e.g. imagine an imported and totally unrelated male of a small breed was mated to all UK females of that breed. All puppies will have a CoI of zero (as parents are totally unrelated) but they will all be half siblings, and mating of half-siblings results in CoI of 12.5% (and the same rate of inbreeding across that generation).

I have questions about my breed report. How do I help my breed and what support can I receive or tools can I use to help me with my own breeding lines?

There are a number of tools available on The Kennel Club’s website which may be of assistance, including Inbreeding Co-efficient (CoI) calculators, our Health Test Results Finder, which also contains and more accurate indicators of genetic risk for hip and elbow score (estimated breeding values) for some breeds. 

You can also email our health team for information and advice: