Linebreeding vs. Inbreeding – Let’s be perfectly clear.

Note: This topic was prompted partly by IPFD's participation in the Canine Health Summit put on by Embark Veterinary. See our Q&A article on breeding and genetics topics here.

Inbreeding is the mating of related individuals – that is those who have one or more relatives in common. Linebreeding is not simply a form of inbreeding – it IS inbreeding.  

How close that inbreeding is depends on the selection of individuals within that line.  

Linebreeding/inbreeding - by definition - reduces genetic diversity. 

By how much depends on the closeness of mating pairs and the time/number of generations over which the process is repeated.

It is scientifically proven and widely recognised that inbreeding/linebreeding in dogs has led to:

• An increase in the prevalence of inherited disorders
• A decrease in viability/ longevity
• A decrease in reproductive ability (reduced fecundity, decreased litter sizes, etc.)
• The loss of genetic diversity (i.e. decrease in genetic variation)
•  Studies in various species have shown that inbreeding can also result in:
  • developmental disruption, higher infant mortality 
  • a shorter life span and reduction of immune system function. And it is known that impaired immune function can lead to higher rates of cancer, and a myriad of other issues for skin, digestive, and other systems as well as resistance to infectious agents.

Collectively, these effects of inbreeding/linebreeding are called inbreeding depression. Importantly, inbreeding depression increases as the extent of inbreeding/linebreeding increases.  For example, see Inbreeding depression reduces litter sizes in golden retrievers.

I was asked by a breeder at a lecture series, during a talk on breeding ethics, “What level of inbreeding is okay?  Where is the cut-off for ‘too close’?”  I asked the breeder, “Why do you want to do inbreeding/linebreeding? Do you think that will improve health and longevity?”  Those at the seminar had previously stressed that their goal was to produce healthy, long-lived puppies.  I suggested that inbreeding/linebreeding is generally done to fix certain characteristics in a line - ‘desirable’ characteristics.  But what is desirable? Generally, linebreeding is on physical, conformational, or structural characteristics.  Not necessarily, and perhaps rarely, primarily on characteristics associated with health and longevity. Therefore, the latter are unlikely to be achieved by inbreeding/linebreeding and will be reduced by the associated decrease in diversity.
From the AKC article above: ”Want to add new things to improve the quality of your line? Outcross with a line that has those characteristics you want. Want to be sure to keep the great things you have? Breed closely to get a very consistent litter. When you know what you want, then you can plan to attempt to create the result you desire.”
Maybe it is time to be transparent and honest.  

‘What you see’ in any pedigree breed – appearance, health, length of life – is ‘what you got’ – a reflection of what was selected for – and what was not selected against – by those who bred them. 

If your primary goal is to produce healthy, long lived puppies – and that includes all the puppies in all litters – then you would first and foremost mate only from dogs that are themselves healthy, who come from a long line of relatives that are healthy and long-lived;  from a long line of dogs that have not exhibited the deleterious conditions listed above as arising from inbreeding/linebreeding or any other conditions of significance in your breed.  And you would want to be sure there was a history of great temperaments and breed-appropriate abilities.  You would make sure that all the dogs in your line can see, and breath, and move without impediment, and are able to exhibit natural dog behaviours.  This is not only the art and science, but the common sense of dog breeding.  

Once prioritized on those aspects, eliminate from breeding dogs with genetic or other screening results that indicate that they are likely to pass on detrimental traits.  After that, only after that, you might do some selection on specific physical attributes.  

Is all that easy to achieve?  No, but who ever said that manipulating the development and genetics of a species should be easy?

And, unfortunately, within the breeding populations for many breeds, there may be rather few individuals who are as acceptable as is described above.

If what you want is, however, consistency based on physical characteristics,  then linebreeding and inbreeding may well give it to you.  But, as history has proven, eventually, at a breed level it will often result in the increase in other characteristics, challenges, and issues - not likely to be primarily health and longevity.  

There are many questions about how to measure genetic diversity, how to determine it – really what is genetic diversity and how do we achieve it?  There are no simple answers to those questions.  There are major challenges in some breeds where characteristics associated with health and welfare problems are ‘fixed’ and can not be bred away from within a closed population. 

Here are some facts about genetic diversity: 

• Genetic diversity cannot be achieved by selecting for conformity and consistency in appearance or specific physical traits – i.e. for a lack of diversity in appearance and physical characteristics.  In other words, a lack of diversity in observable characteristics reflects a lack of genetic diversity.
• Eliminating from breeding dogs with variations in ‘minor’ characteristics that are not associated with poor health or function, e.g. many coat colour varieties that are simply deemed as undesirable, will reduce genetic diversity. (Recognizing that a few coat characteristics reflect deleterious mutations or fads. Interested in challenges around coat colours?  See Ian Seaths' blog: Systems Thinking and Non-standard Breed Colours.)
• Diversity will always be reduced by inbreeding and linebreeding – the closer the mating pairs, the longer the linebreeding/inbreeding is followed, the bigger the effect.
• The ability to increase genetic diversity by breeding only within a closed population is limited,  However, calculated measures of diversity may be inaccurate within a registry if only a small proportion of the available population is bred or included in calculations.
• Pedigree-based calculations of Coefficients of Inbreeding (COIs) – especially on a limited number of generations – underestimate inbreeding compared to genomic based COIs as they are based on average/ predicted inheritance of genetic material from ancestors, rather than the actual case for an individual dog.
• Measures of genetic diversity are one more tool to inform breeding for the ‘Big Picture’. (See my blog: The Big Picture - in the Dog World as a Whole and for your next Breeding Decision.)
   

What can be done for your breed?

There are increasing numbers of breed-specific research studies on genetic diversity. The whole picture for a breed cannot be understood by one calculation or measure done on a limited group of dogs. For an example of a broad-based, global picture, read, e.g.: Genomic diversity and population structure of the Leonberger dog breed.  You do not need to focus on the details of the technology and methodology to get the main messages from this paper.  For example: "Conclusions: The increasing size of the Leonberger population has been accompanied by a considerable loss of genetic diversity after the bottleneck that occurred in the 1940s due to the intensive use of popular sires resulting in high levels of inbreeding." and "The breed has predispositions to neurodegenerative disorders and cancer, which is likely due in large part to limited genetic diversity."

Aaron Sams, expert in genetic diversity at Embark, was responding to a question on the summit that again, was someone knowing how much inbreeding was okay, what level was okay in a breed.  And among other sage advice he said that ultimately the breeders and breed club must determine what they can 'tolerate'.  And I would add that goes for all aspects of health and lack of it, and longevity issues.

Bottom line – you get that for which you select, not that for which you wish. 
Let’s all be perfectly clear.

Other resources on DogWellNet.com:

1. Genetics Vocabulary - Glossary of Terms

2. Relationships between genetics, breeding practices and health in dogs - Grégoire Leroy (France)