The Mystery of the "Bad Bite"
Elementary, My Dear Watson
by Diane Klumb
Anyone who knows me at all probably also knows how totally excited I am by the ability of molecular genetics to solve the mysteries inherent in the breeding of purebred dogs. In addition to allowing us to actually "breed for improvement" instead of just blithely throwing the term around, I firmly believe that if used wisely, this new store of knowledge represents out best hope for both preserving the sport of dog breeing for future generations, and for fending off our own personal Professor Moriarty in the guise of Ingred Newkird & Co. But actually using this new knowledge to our benefit, and to the benefit of dogs, often requires us to discard long-held and long-cherished beliefs.
Realizing that something we were taught years ago (and in many cases have passed on to the next generation of breeders) was based on an incorrect assumption, and may actually be flat-out WRONG, can be a difficult mental pill to swallow, and some people just can't seem to do it.
For others, it provides an "Ah-HA!" moment, when the seemingly inexplicable suddenly becomes clear.
One such moment for me occurred a few years ago, when I learned that prenatal disruption (via genetics or environment) of a regulatory gene with the delightfully improbable name sonic hedgehog (SHH) often results in asymmetry, where the two sides of a dog don't exactly match. (It's a lot commoner than you'd expect, actually, and occurs in people to varying degrees as well. And symmetry in people has been linked to both beauty and longevity. Probably true in dogs as well.)
More to the point, an asymmetric dog will invariably crab, as he has longer reach and more drive on one side than the other, causing his forward progress to eerily resemble that of a '63 Ford Fairlane with a bent frame. Yet stacked in profile the dog displays flawless balance, which has confounded judges and breeders since time immemorial.
When I shared that discovery in a column a few years back, an amazing number of judges who read it made a point of telling me that it was an "Ah-HA! moment for them, too. (One told me that now whenever she sees a dog crabbing, she checks the elbows on both sides, and one is invariably set higher on the ribcage than the other.) An old dog show mystery solved by molecular genetics. Cool.
I had another of those "Ah-HA! moments recently, when I stumbled upon a fascinating research paper while looking for something else entirely. (Happens to me all the time.)
It seems that scientists have discovered that the size and shape of the mammalian mandible (or lower jaw) is controlled by a surprisingly large number of genes - over 15 have been identified to date.
A little more digging revealed that an equally large number are involved in the development of the maxillary complex, or what we refer to as the upper jaw.
The kicker is......they are different genes, and inherited pretty much independently. Which means, in terms expressed as simply as humanly possible: A DOG CAN INHERIT HIS UPPER JAW FROM ONE PARENT, AND HIS LOWER JAW FROM THE OTHER. Ah-HaH! Another dog-breeding mystery solved, and a long-cherished belief laid to rest.
Putting this into an everyday breeding scenario, here's what too often happens. A young health-screened dog of quality with a magnificent head is widely used by breeders on bitches whos heads could use some improvement--depending on the breed standard, their muzzles could be a little shorter, or a little longer, or maybe a little more or less refined.
But rather than the overall improvement in the first generation breeders are hoping for, they get maybe one nice bite (if they're lucky and depending upon what the bitch's parents looked like) and a basketful of "bad" bites. (What constitutes a bad bite varies from breed to breed, of course.) Soon the word goes round that this lovely-headed dog "throws bad bites" and his stock drops faster than Lehman Brothers. Happens all the time.
And now we learn that it wasn't his fault at all, poor guy. Breeders have been laboring for years under the misconception that an off-bite is the result of an AR gene, and that some dogs are carrying a recessive gene that causes them to "throw bad bites." I've heard it said a thousand times over the years, and so have you.
But it is simply NOT TRUE. Turns out there is no single AR gene for an undershot bite, or an overshot bite, either. There are literally dozens of genes involved, all inherited more or less independently.
So, from this day forward (unless you are one of those people now recognized as incapable of changing a long-held opinion in the face of new evidence due to insufficient activity in the anterior singulate cortex and I'm wasting my time here) we can all stop blaming the poor stud dog.
What is actually happening genetically is this: Given Mendel's Law of Independent Assortment, which is still scientifically valid after all these years, a percentage of the pups from an "unlike-to-unlike" breeding in the head department will inherit a larger percentage of the genes for a longer mandibular (under) jaw from one parent, and a larger percentage of the genes for a shorter upper maxillary (upper) jaw from the other, resulting in bites that are undesirable per a particular breed's standard. NEITHER parent is to blame - malocclusions of the jaw, we now know, are polygenic.
Now, hopefully most of us already understand that there is a huge genetic difference between a MALOCCLUSION OF THE JAW and MISALIGNMENT OF INCISORS, which cause a reverse scissors bite in a dog whose jaws align according to the standard, and whose "puppy bite" is often perfect. Misalignment of incisors is usually caused by no more than the particular timing of the eruption of the individual permanent teeth - if it is off, the upper incisors will force the lower ones out, resulting in a reverse scissors. (That's why it's correctable with mere pressure.) There's no sense blaming this one on either parent, either:
Research has shown there are more than FIFTY different genes that influence the development, and timing of eruption, of teeth.
Some of these genes, it turns out, are involved in other processes and also code for traits that we've actually selected FOR over the years---the MITF gene, for example, which is involved in pigment development (parti-colored dogs are parti-colored because they carry a mutation on this gene) is also involved in toogh development and timing of eruption, which is likely why the parti-colored pups in a litter often get their teeth later than their solid-colored brethren. The RSPO2 gene is also involved in tooth development, and a mutation on this one is responsible for canine head furnishings. (And that's just two off the top of my head- no doubt there are dozens more, as we now know that genes "multi-task.)
The route to overall improvement in bites within a breed
IS THE SAME ROUTE THAT HAS REDUCED HIP DYSPLASIA
in several breeds over the last few decades ----
SELECTION.
This probably explains why wolves -uniformly long-muzzled, solid-colored, and generally free of head furnishings - rarely display the anomalies in dentition that plague purebred dogs.
Now, I'm NOT suggesting for a moment that we should be trying to put a ''wolf head" on all our dogs, or to make them all solid-colored or clean faced-- to do so would seriously affect breed type in probably two-thirds of them, and not necessarily for the better.
What I AM suggesting is that simply understanding that malocclusion of the jaw and misalignment of incisors both appear to be polygenic, rather than the result of a single recessive gene, allows us to make more informed breeding decisions. Breeding a male with a gorgeous head to a bitch who is lacking and expecting the resulting puppies to all end up with his head (and bite) is about as silly as breeding a dog who is OFA Excellent to a dysplastic bitch and expecting the resulting pups to all end up OFA Excellent. No one with half a brain would blame the sire in that situation, because (hopefully!) we now all understand that canine hip dysplasia is polygenic, and represents a threshold characteristic.
The route to overall improvement in bites within a breed is the same route that has reduced hip dysplasia in several breeds over the last few decades--SELECTION. And as the German Shepherd breed has proven conclusively with its OFA ratings, you can do it without sacrificing breed type. Rather than discarding a quality health-screened male with a correct head per his standard who produces off-bites when bred to bitches with poor heads, it would make more sense to selectively linebreed off him, using only those offspring who inherited his head and petting out the rest. After three or four generations of this, the line should be homozygous for his head, the pedigree will have both depth and breadth in that regard, and malocclusions will be few and far between. What we'd be doing is simply combining time-honored animal husbandry practices with knowledge gained from cutting edge molecular genetics. It's the future of responsible dog breeding.
However, refusal to change one's long-held beliefs regarding mode of inheritance (i.e. continuing to believe that there is a single recessive gene for "bad bites" and that a dog who produces one is "a carrier") as new information becomes available to us will untimately result in failure to improve. Why? Because the breeding techniques used to reduce or eliminate the incidence of a trait caused by an AR gene will always be different than those used to reduce or eliminate the incidence of a threshold trait caused by polygenics, where gene testing is not a viable possibility.
And consistently producing sounder, healthier dogs is more important now than ever because, make no mistake about it, the wolf is at our door.
See you at the shows, and remember to have fun out there!
-DK
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Originally Posted by Milehighmisfit
