The Center for Disease Control tells us that in America, nearly 4.5 million people a year are bitten by a dog, and almost half of those bites are delivered to children. Aggression is also a major cause for the euthanasia of otherwise healthy dogs. These days, we have more than a hundred genetic tests available to us to screen dogs for specific kinds of heart disease, eye disease, neurological disease, and many other heritable health issues; we can even test mixed-breed dogs to find out what breeds are in their ancestry. We know that a parent’s personality affects the child. So why can’t we test dogs for aggressive tendencies before breeding them?
The genetics of diseases
In the early 2000’s, the hunt was on for the genes which cause inherited diseases. We had just finished sequencing the human genome, we were starting to sequence the genomes of other animals (the first dog sequence was of a boxer named Tasha, completed in 2004), and we were convinced that the secrets hidden in DNA would open up to us in the next few years. And we did find the answers we were looking for – for some diseases. Some of these were huge wins, allowing highly accurate testing to predict who would be more predisposed to getting these diseases and providing a big boost to treatment research. But others proved harder – cancer, diabetes, many kinds of heart disease. And behavior problems turned out to fall into the second category.
The difference was in the number of genes causing a particular disease. For example, a mistake in a single gene causes the painful and eventually fatal Huntington’s disease, and the discovery of this gene was a major scientific victory. But many other diseases are polygenic, meaning that they are caused by interactions between many genes. Hip dysplasia in dogs is a good example of this kind of disease. A dog with hip dysplasia may have any number of small genetic problems, each of which adds just a little bit of risk for dysplasia; on their own or in small groups, these problems wouldn’t result in bad hips, but once enough of them accumulate in a single individual, the risk of hip problems rises dramatically. The tools that we use to hunt for genes which are associated with particular physical problems struggle with these sorts of associations. It’s difficult to find an association between a gene and a medical problem when that gene is only present in some, but not all, of the animals who have the disease, and when its effects are greatly modified by the effects of other genes in the mix.
It’s not just genetics
Many of these polygenic diseases weren’t only caused by genetics, making the hunt even harder. Heart disease, for example, is influenced not just by a genetic predisposition, but also by a person’s environment, including diet, exercise, and stress. Behavior, of course, is influenced hugely by environment: how was a puppy socialized? How is she managed as an adult? Has she had any particularly bad experiences? Environmental factors are a major component of any behavioral disorder and are very hard to study. Researchers find that even mice who are extremely genetically similar to each other, and raised in identical environments, show personality differences. What seems like an identical environment to a human might have important differences to a mouse – maybe a littermate was a bully, maybe the position of a favorite hiding place provided a safe haven. These small differences can have significant effects on personality.
How do you define “aggression?”
Classifying diseases is difficult, especially for behavior issues. Aggression is a great example here. It comes in lots of types: territorial aggression, fear aggression, food guarding or toy guarding; aggression against other dogs, against humans in the family, against strangers. These different forms of aggression are probably all caused by different factors, both different genes and different environmental changes. In order to study a particular form of aggression, we have to know that all of the dogs in our study have the same kind of aggression. But we haven’t completely defined the different classes of aggression, because we don’t completely understand its causes – which is part of why we’d like to find the genes affecting it in the first place! It’s a vicious circle which researchers have difficulty breaking out of.
So where does that leave us? There are no genetic tests for aggression today. Although you can test your dog to find the breeds in her ancestry, you can’t know what personality traits each ancestor has passed on. In the end, we are still where we started: knowing that early socialization and good management are the best ways to keep dogs behaviorally healthy. It remains true that the best predictor for whether a dog will bite is not the dog’s breed, but the environment he’s kept in: a dog who lives with a family is much less likely to bite than one who lives outside on a chain.
Will we ever crack the nut of the genetic causes of aggression and other personality traits? I hope so! We are just at the beginning of a golden age of genetics. We are learning more and more about how environmental differences affect animals at a genetic level, and we are realizing that genes aren’t the whole story as we originally thought. I believe a paradigm shift is coming in the next few decades which will open new doors to allow research into these questions. We aren’t ready to find the answers yet, but I hope that soon we will be.
Taras Verkhovynets © 123RF.com, Cathy Yeulet © 123RF.com
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