What do the terms inbreeding and linebreeding mean?

What is inbreeding?

Inbreeding is the mating of related individuals who have one or more ancestors in common [1]. Linebreeding is a form of inbreeding.

Inbreeding reduces the genetic variation within that breed or population. Inbreeding has been a common practice in companion animals (and other domesticated species) as it increases the likelihood that animals will share desired traits such as their physical characteristics because they are genetically similar [1, 2]. Inbreeding has been the foundation for the development of pedigree(pure bred) breeds as it results in offspring who are predictably similar, because of the fact that their genetic similarity maintains specific traits from a common ancestor (i.e., the genetic similarity of dogs of the Labrador breed means they look the same; they look like a Labrador) [2, 3].

However, the effects of inbreeding accumulate within closed populations (e.g., within purebred breeds) and negative consequences can result from breeding parents who are too genetically similar. Inbred animals are more likely to have genetic defects and inherited diseases [2, 4], which can be extremely detrimental to their health and welfare. Examples include Polycystic Kidney Disease in Persian cats (leading to kidney failure), hypertrophic cardiomyopathy in Maine Coon cats and Mitral Valve Disease in Cavalier King Charles Spaniels (both diseases leading to heart failure), and spinal abnormalities in British and French Bulldogs, Boston Terriers and pugs.

In addition, inbreeding has other negative effects on the animals’ health (called inbreeding depression) including fertility problems, reduced survival and fitness of offspring, and weakened immune systems [2, 4].

What is close inbreeding?

Close inbreeding is the intentional mating of close relatives [3]. This includes mating first-degree relatives (e.g., full siblings [full brothers and sisters in human terms] and between parents and offspring) and second-degree relatives (e.g., grandparents and grand-offspring, half siblings, and related individuals equivalent to uncles/aunts and nephews/nieces, and double-first cousins in human terms).

Breeding from closely related animals predisposes the offspring to genetic defects or inherited disorders and inbreeding depression [3]. Close inbreeding (breeding first and second-degree relatives), substantially increases the risk of genetic defects or inherited disorders and inbreeding depression and these occur at a faster rate, compared to inbreeding of less closely related individuals [3].

What is linebreeding?

Linebreeding is where breeders frequently choose to mate parents who have a common ancestor, but who are not as closely related as in close inbreeding (e.g., not first-degree and second-degree relatives) [4]. The aim is generally to improve or maintain specific traits within the breed [4]. As a form of inbreeding, linebreeding is still associated with the risks to animal health and welfare described above.

Coefficient of Inbreeding

One method which is often used to measure the degree of inbreeding, or how closely related the parents are, is the Coefficient of Inbreeding (COI). COIs measure the genetic similarity looking back over generations of an animal’s pedigree (similar to a family tree for humans), not just whether the parent animals are close first and second-degree relatives. The higher the COI, the higher the probability that the parents are genetically similar and, consequently, the more likely it is that there will be problems in the offspring related to the accumulation of inbreeding effects [1] (e.g., genetic defects, inherited disorders and inbreeding depression).

The European Union Responsible Dog Breeding Guidelines and the Dog Breeding Reform Group state that breeders should avoid breeding from individual dogs whose combined coefficient of inbreeding (the combination of both parents), measured over at least 10 generations, is greater than 6.5% [5]. The European Union Responsible Cat Breeding Guidelines and the Governing Council of the Cat Fancy Breeders state that breeders should aim to breed from individual cats whose combined coefficient of inbreeding (the combination of both parents), measured over at least five generations, is below 10% [6].

In addition to assessing the COI or other measures of the degree of inbreeding, all breeding animals should undergo a check by a veterinarian to assess their health and suitability for breeding. Furthermore, breeding animals should undergo testing for any known inherited diseases and welfare problems in that breed (see below) and the results of these tests should be considered in the assessment of their suitability for breeding.

What is the RSPCA’s position?

The evidence demonstrates that inbreeding has significant negative impacts on animal health and welfare. Therefore, the RSPCA is opposed to the inbreeding of companion animals including first-degree (e.g. father to daughter) and second-degree (e.g. grandfather to granddaughter) matings, as this increases the incidence of inherited disorders and compromises immune system function, both of which adversely impact the health and welfare of companion animals. People who breed companion animals must prioritise the health, behaviour, and welfare of both breeding animals and offspring. Parents should be chosen who are not closely related and are physically and behaviourally sound.

What should I consider if I’m thinking about adding a purebred dog or cat to my family?

If you are interested in a particular breed of dog or cat, be aware that different breeds are predisposed to different inherited disorders, and health and welfare problems. Breeders also have different practices; for example, some practice close inbreeding while others do not. The breed you choose and the breeder and their practices will significantly influence your future companion animal’s health and welfare.

We recommend that you:

• Find out if the breed you are considering has known inherited diseases and welfare problems. You can find breed information on inherited diseases and welfare problems in these resources:

  Dogs – UFAW

  Cats – UFAW

  Inherited Disorders in Cats | International Cat Care (icatcare.org)

• After finding out what known inherited diseases and welfare problems there are in the breed, ask the breeder whether they test for these and what the results were for the parents and, if applicable, the puppies or kittens. Speak to your veterinarian about the results and the risks and impacts of any inherited diseases and welfare problems.
• Ask the breeder if the puppy or kitten’s parents are first-degree relatives or second-degree relatives. If they are, this carries significant risks of negative impacts on animal health and welfare and should be avoided.
• Find out if the breed you are looking at has health and welfare problems specifically related to the way they are bred to look (i.e., exaggerated features that are part of the pedigree breed standard). Exaggerated features can cause dogs significant pain and suffering so be sure to find out about the problems that are known to occur in that specific breed or breed type. Examples include brachycephalic dogs (e.g., Pugs, Boston Terriers, French Bulldogs, British Bulldogs) and Dachshunds, as well as Scottish Fold, Manx, and Persian cats. For more information, see Love is Blind and the linked resources below. Speak to your veterinarian about the risks and health/welfare impacts of any exaggerated features.
• Ask what action the breeder takes to prevent inherited disorders and avoid exaggerated features in their animals. Every breeder has an important responsibility to ensure their animals live a healthy, happy and long life.
• Ensure that you are buying from a responsible breeder, who complies with the relevant legislation and standards of care in their jurisdiction and make sure you meet the puppy or kitten and the parents (or at least the mother) before you buy the puppy or kitten.
• Take a look at our tips on how to find a good dog breeder, how to look safely for a new pet online, and red flags to look out for.