Wednesday, August 05, 2020

Inbreeding Has Broken Bull Terriers

From Popular Science comes this note about perilous health of the very heavily inbred bull terrier.

Bull terrier owners might feel some sense of relief upon hearing that a team of international researchers have identified a genetic mutation behind lethal acrodermatitis. Baby bull terriers with LAD don't grow well, have deficient immune systems, and develop skin lesions around their paws. They often die before their first birthday. 

But here’s the problem: at this point, we can’t really do much about it. 

The reason why lies in the very first sentence of the paper, which came out on Thursday in the journal PLoS Genetics. "Lethal acrodermatitis is a genodermatosis with monogenic autosomal recessive inheritance in Bull Terriers and Miniature Bull Terriers," the study reads. Maybe that doesn't sound like much to you, but it means a lot to canine genetic researchers — that's people who study dog genetics, not dogs who study genetics — because it highlights immediately why this disease is much like the vast majority of other diseases that plague purebred dogs. It's autosomal, meaning it can occur on any chromosome (not just the sex-determining ones), and it's recessive, meaning you need two copies for the disease to manifest. 

"About 70 percent of the genetic disorders in dogs are autosomal recessive," explains Carol Beuchat, Scientific Director of the Institute of Canine Biology. "And the reason why is because a recessive mutation doesn't hurt you if it's not expressed, so it doesn't get selected against." If everyone who carries a mutation suffers the associated ill-effects, they'll be less likely to reproduce and the mutation won't spread through the gene pool very much. But recessive genes can proliferate more pervasively, because carriers with one copy can lead perfectly healthy lives and have lots of children, some of whom will also carry the recessive gene on to their own offspring.

.... If two siblings have children together, they have what's called an inbreeding coefficient of 25 percent. That means that their offspring are homozygous (have two copies of the same gene) for 25 percent of their DNA. About half of all dog breeds have an inbreeding coefficient above 25 percent. And bull terriers? They're the second-most inbred dog of all. Their coefficient is around 60 percent. Generation after generation has compounded the problem, which is how they've got a coefficient ever higher than pure siblings can produce.

So it shouldn’t really be a surprise that the breed has accumulated a few genetic inconveniences along the way. Bull terriers are also prone to breathing problems, heart and kidney disease, deafness, and knee issues.

.... Bull terriers are already at a 60 percent inbreeding coefficient, which means those recessive mutations have accumulated already. They're not going to go away magically, and they certainly won't disappear without raising a few more diseases to take their place. The only solution is to stop inbreeding. Bull terriers will eventually die out as they become increasingly inbred. Populations with finite gene pools simply don't survive. To save the breed, you have to make it impure. 

There aren’t many breeders who want to do that, though. The Norwegian lundehund is almost entirely extinct because they’re so inbred, and yet the effort to save them by outbreeding to other Norwegian dog varieties only comprises a handful of breeders. Most lundehund owners outright refuse to allow their dogs to mate with anything but another lundehund. It’s the same story with many other breeds.

The majority of dobermans now succumb to the same heart condition. Bernese mountain dogs today have little hip dysplasia, but die of cancers that have become commonplace.

You know what's sad?  This is a breed invented by an Irish dog dealer who knitted it up by crossing half a dozen different canine types, and it is a breed that has radically changed appearance over the last 150 years. There is, literally, nothing to "preserve" and nothing to ruin by outcrossing.  You could keep this dog's personality AND make it healthier.  But will that be done?  Nope.

Changes to the Bull Terrier nose:  1930, 1950, 1980.

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