Dog Health

Polygenic, modifiers, environmental triggers and other meaningless words

If you came to me right now and told me that your dog had been diagnosed with Hideous Nasal Syndrome That Nobody Has Ever Seen In Dogs Before And The Vet Gasped And Fell Over, the first thing I’d say to you is “Well, I can tell you immediately that it’s a polygenic trait, likely with unknown modifiers, and something environmental triggered it.”

If you came to me and said your dog had been diagnosed with an allergy, I’d say, “Well, I can tell you immediately that it’s a polygenic trait, likely with unknown modifiers, and something environmental triggered it.”

If you came to me and said that your dog had been diagnosed with osteosarcoma, I’d say, “Well, I can tell you immediately that it’s a polygenic trait, likely with unknown modifiers, and something environmental triggered it.”

If you came to me and said that your dog had been in a car accident and had fractured his femur, I’d say, “Well, I can tell you immediately that it’s a polygenic trait, likely with unknown modifiers, and something environmental triggered it.”

Getting the picture?

In medico-speak, there is one completely safe thing you can say about any disease, any disorder, because it’s true of all of them and doesn’t even need to be verified: “Well, I can tell you immediately that it’s a polygenic trait, likely with unknown modifiers, and something environmental triggered it.”

Dog breeders hear those words and they assign majestic significance to them. We have a few disorders that are largely governed by single genes – PRA is one of them – and we WANT to have a kind of genetic determinism attached to every single problem we have in dogs. So when someone says that a disorder or disease is polygenic, we figure that means it’s just like PRA, meaning that we just have to identify the genes and then breed away from them.

In fact, those phrases – polygenic, unknown modifiers, environmental triggers – are the same thing as saying that “it happened to a living creature.” Every single thing that occurs in a living body, from an injury to a cancer to an allergy to a crazy rare disease, is the action of lots of genes and the modifiers of those genes combined with an environmental trigger. And yes, even the accident – your dog may have broken a femur where my dog would not; my dog might have broken a rib instead, because her genes for bone formation and the tensile strength of the bone are different from your dog, and her diet has been different.

I said in the title that those words were meaningless – that’s perhaps the wrong word to choose. It’s more that they’re IMPACTLESS. Knowing that whatever it is that went wrong is polygenic gives us absolutely nothing to do; it gives us no more information than we had before we knew that. It’s in many ways very cruel, though I know it’s well-meaning, for researchers to imply that they actually know anything about a disorder when they say it’s polygenic or that it’s a genetic tendency combined with an environmental trigger. It makes breeders think we know something, or are on the cusp of knowing something, when nothing has changed.

Learn to ignore those words. Instead, look for studies that identify REAL genes or real proteins. If they don’t exist – and let me tell you that they do not exist for hip dysplasia, autoimmune disorders, most canine cancers, and so on – then realize that we don’t actually know anything about that problem than we did before those words existed. Those words have no power and no instructions attached to them; they deserve none of your respect.

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13 Comments

  • Reply Jeff Welch October 5, 2010 at 11:40 pm

    Actually Joanna, they are not meaningless and are often used in the context of inheritance that does not follow simple Medelian genetics.

    • Reply rufflyspeaking October 5, 2010 at 11:43 pm

      They are meaningless in terms of their impact for breeders. Among researchers, they seem to be a shorthand for “we don’t really know.”

  • Reply Joanna Kimball October 6, 2010 at 12:40 am

    NOTHING follows simple mendelian genetics. Except maybe whether peas are wrinkled, and even that is modified by various other genes that control when the wrinkling begins, how deep the wrinkles are, what colors are expressed in the wrinkles, how strongly the wrinkling is inherited, etc. Mendelian inheritance is fantastic in terms of offering a simple way to understand that traits are carried and expressed based on how genes meet, but all it really means is that some traits have a single gene that is very strongly conserved in the trait and seems to be very strongly responsible for it (like PRA’s faulty protein encoder). But no traits are ONLY one gene and no traits exist apart of environment. “Polygenic” – when no genes have actually been identified or even implicated – just means “we don’t know.”

  • Reply Jeff Welch October 6, 2010 at 1:40 am

    Sigh; “Her eyes closed” (Star Trek I believe). Don’t over complicate life, life will do that on its own.

  • Reply Joanna Kimball October 6, 2010 at 2:40 am

    My point is this: When somebody says “It’s polygenic” – one of the favorite lines attached to things like hip dysplasia – the response should be “Oh, they discovered the genes responsible?” The answer will be “No.” The reason the answer can be no, and the phrase “it’s polygenic” can still be true, is that ALL disorders, diseases, injuries, and reactions are polygenic. It’s not actionable information and it shouldn’t be either a comfort to breeders or a caution to them. Too many breeders hear “polygenic” and think we know anything, which we don’t.

  • Reply elaine October 6, 2010 at 6:37 pm

    My layperson’s understanding of “polygenic” is “We have reason to believe there are multiple genes that contribute to this condition and we don’t know for sure which ones they are (and quite possibly will not find out in your lifetime).”

    But I’m pretty old compared to some readers.

  • Reply Jeff Welch October 7, 2010 at 2:40 pm

    Let’s go back up a bit. You are *not* convinced that Cardigan PRA (blindness) is due to the mutated phospodiesterase gene? Did I understand that correctly?

  • Reply Joanna Kimball October 8, 2010 at 8:39 am

    Sorry, I lost this thread and didn’t realize you’d responded. I believe, because the research is very good, that what we call the PRA gene is strongly responsible for the disease. There are, always, other things taking place as well, which is why there’s a variation in age of onset and severity of symptoms and so on. But we’re lucky that the PRA picture is as clear as it is.

    What I think is a HUGE mistake is that breeders hear the word “polygenic” and in their own heads translate that into “it’s just like PRA, except that we have to find two genes instead of one.” And meantime we all have a responsibility to “breed away from” those genes. They don’t understand, and it is not clearly spelled out, that most often the reason “polygenic” gets slapped on a disease before there’s been any actual research to establish which genes are or are not involved is that pedigree studies have shown such a WEAK relationship between family and disease.

    Every genetic researcher wants to see a pretty pedigree with an obvious single dominant or recessive gene, or a sex-linked trait. When they don’t see that, when they see no obvious relationship at all, they say “hmmm… must be polygenic” – again, a great guess because everything is polygenic to a greater or lesser extent. In the researchers’ minds, calling it polygenic doesn’t mean anything except that it’s NOT a simple recessive or dominant. But breeders hear that and attach meaning to it, and begin making breeding decisions based on it, and use it as a bludgeon to attack other breeders who don’t make breeding decisions based on it.

    Hip dysplasia is a great example of how wrong this has gone – we have many breeders saying that the reason it exists in the population is because of irresponsible other breeders. They feel that they have permission to blame others because “it’s polygenic,” as though that means we know the genes and are just ignoring them and chuckling and twirling our mustachios as we breed dysplastic dogs. But when you look at the research, you get studies like this: http://www.ncbi.nlm.nih.gov/pubmed/19959383.

    Does that mean that somehow irresponsible breeders are attracted to longer-bodied dogs? Longer body length brings out all the evil fanciers?

  • Reply Jeff Welch October 8, 2010 at 11:41 am

    Okay, you are not *totally* convinced that the mutated phosphodiesterase gene is responsible for retinal atrophy caused blindness. Also you believe that some of the blind dogs were more blind that others and that PRA in Cardigans is polygenetic. I see why you would have a understanding DM and HD research.

    I don’t think most breeders are as uninformed as you seem to believe and most go out of their way to educate themselves about the breed. Unfortuantely, new fanciers, when they use the internet come across arm chair reports that are inaccurate and are leading. They may not have the abliity to sort fact from blather.

    The last is an interesting report although not something totally unexpected if you remember your fun physics facts and the square/cube law. This may simply be a phenotype caused by environmental factors (laws of physics).

  • Reply Jeff Welch October 8, 2010 at 12:39 pm

    Sorry, above should read: “I see why you would have a *probelm* understanding DM and HD research.” That’s what I get for typing before the first cup of coffee ;-)!!!

  • Reply Jeff Welch October 9, 2010 at 12:39 pm

    Joanna, I keep going back to PRA because you are the first (and I hope only) breeder in my experience that has said they do *not* believe that the phosphodisesterase mutation rcd3 is definitely responsible for PRA associated blindness in Cardigans.

    Given your opinion of the PRA test, it is easy to see how you would criticize tests and health assessments where there actually are knowledge gaps. As a breeder I find it disturbing that you, also a breeder, would discount the tools provided, even if imperfect. Yes some vocal individuals way over state the efficacy of tests like DM, but it is just as uninformed to say the test is useless. I guess we will have to agree to disagree.

  • Reply Joanna Kimball October 9, 2010 at 4:39 pm

    Jeff, where on earth do I say that the mutation is not responsible for PRA? So far I count three places in just the thread above where I say it IS.

  • Reply Joanna Kimball October 9, 2010 at 5:39 pm

    Also, for what it’s worth, I use every test. Everything is x-rayed, everything is CERFed, I know the DM status of every breeding bitch. What I do NOT do is tell owners that I’m doing anything more than whistling in the dark. I’m pretty sure, and the evidence is pretty dang good, that using OFA is “imperfect” like using a fork to build a house is “imperfect,” but I STILL look at every single set of hips. This isn’t about my decisions, which are in line with the fiercest of test advocates. It’s about whether it’s OK to criticize breeders who don’t, and about whether it’s OK to imply to puppy buyers that we’re somehow protecting them from hip dysplasia by OFAing, or from being paralyzed by knowing DM status.

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