The genetics of designer dogs and XL bullies

When does inbreeding become a problem?
14 November 2023

Interview with 

Cathryn Mellersh, University of Cambridge


Bulldog puppy


Whilst our infatuation with selectively breeding animals goes back a long way, and the animal that may well have kicked it off was the dog. Whilst they were initially bred to help protect us, nowadays our canine counterparts are perhaps more of a fashion statement than a means of conserving livestock, hence the emergence of the phrase ‘designer dogs.’ But what are the side effects of our striving to create so called pure breeds, and where do we draw the line, as has happened with the UKs ban of XL Bullies. Cambridge University's Cathryn Mellersh.

Cathryn - Humans have been probably selectively breeding dogs for possibly as long as 10,000 years ago. So somewhere between 10 and 5,000 years ago, humans sort of started to actively breed dogs to help them do the jobs they wanted them to do. Hunt primarily, guard, maybe herd sheep, and also serve some companion purposes as well. But I think it's safe to say that selective breeding has not been done as intensively in the past compared to what it's been done within the last maybe 200-300 years. When humans have really sort of ramped up the selective pressure to produce a far greater number of dog breeds, increasingly designed to not only behave a certain way, but to look a certain way as well.

Will - When you try and create a more and more specialised dog for a task you want to complete, it seems you are going to get a smaller and smaller pool of dogs you can choose from when you come to breed these. So what kind of issues arise when you have a smaller gene pool to select from?

Cathryn - So to make the fastest progress when you are selecting for a particular trait, you can make progress very quickly if you breed dogs that are closely related to one another because they tend to share the characteristics that you are selecting for. And that, by definition, will lead to a decreasing gene pool. And that means that the dogs within that population, their genetics are increasingly similar to one another, and that means that they can't adapt to new changes that might come along. And they tend to have an increase in what we call regions of homozygosity within the DNA. And that can lead to an increase in deleterious genetic mutations or genetic variants that cause a particular disease. And selecting against those deleterious mutations becomes increasingly difficult when the dogs within the gene pool, their DNA becomes more or less the same as one another.

Will - Seemingly to counter this then there's been, as I perceive it, an increase in two different species of dog being cross-bred so that you get perhaps a more genetically diverse individual at the end of it. But is that always the case?

Cathryn - I think the idea behind these designer breeds, the cockapoos and the labradoodles, is yes, just as you've said, you get the best of the different breeds. I think they are perceived to be healthier because they are 'cross breeds'. But I think there's evidence that some of these dogs are just as inbred as what we think of as the pure bred dogs because the breeders will take, for example, a cocker spaniel and breed it to a poodle, and that's what we would call an F1 cockapoo. But if that dog is then bred to a purebred poodle or a pure bred cocker spaniel, then you start to get inbreeding. And I think that some of these designer dogs that are bought are probably just as bred as some of the purebred dogs. But my concern is that the people who are breeding these designer dogs are maybe not taking advantage of health testing to the extent that the purebred dog breeders do. So they're not having the hips tested or elbows tested or eye examinations or doing some of the genetic tests that the conscientious breeders of the purebred breeds are taking full advantage of. So I think they can be a bit of a misnomer that they're crossbreds, many of them are not crossbreds anymore.

Will - And so where do you think the action lies in this case to try and ensure that there's a greater genetic diversity when it comes to owning one of these dogs?

Cathryn - I would like the puppy buying public to just become generally more aware of health issues that are associated with different dogs, different breeds of dog, different types of dog, and just get a lot more savvy about doing their research before they buy a dog. I think most people do more research when they're buying a new fridge than when they're buying a new dog. The public, I think, is where this situation can change. And if, when they're buying a puppy, they're not afraid to ask the breeder of that puppy what health checks the parents had. But a conscientious breeder will mind you asking questions about how the puppy was bred and the health checks that the parents had in the same way that a conscientious breeder will ask the puppy buyer what kind of home they are going to be able to provide.

Will - I'm sure you're sick to death of it, but I couldn't come here and not ask you about the XL bully story developing. With the UK government banning it and certain dog organisations saying that the ban is unreasonable, what genetics take on this? Is there any point in attempting to ban a breed if you can just recreate it? Or is there a genetically distinct XL bully that could be banned in the first place?

Cathryn - I think the XL bully is a considerable problem for our society. They've been responsible for just over half of the dog related human deaths in the last couple of years. So I think it does need to be addressed, and I think it's very naive to ignore the fact that behaviours are inherited. You know, we are not surprised when our Labrador retriever puppy retrieves a ball or our whippet chases a rabbit. There's very good evidence that some breeds are likely to be more aggressive or have genetic tendencies to be more aggressive. And we have to acknowledge that. I think banning them is probably not the way to go, necessarily. I think there's evidence that banning breeds of dog has not decreased the number of dog bites and dog attacks, for example. And there's the danger that when you ban something, you make it something that's good to have and drives the breeding of these dogs underground to some extent. Personally, I'm more in favour of encouraging the owners of these dogs to take a responsible stance, and being muzzled in public is not a terrible thing to ask owners. If the dog is trained to wear the muzzle, it's not a big deal. The dog will be perfectly happy wearing one. So I think I'm not particularly in favour of banning them outright, but let's put some measures in place that increase the responsible dog ownership aspect of keeping an XL bully.


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