Genetics of aggressive behaviour in Golden Retriever dogs

Abstract

Dogs have been living in close proximity to humans since the last Ice Age. Like their progenitor the grey wolf, dogs may respond with aggressive behaviour to certain stimuli. This is natural behaviour in the majority of cases. However, canine aggression can develop into a dangerous problem. There is individual variation in the tendency of dogs to display aggressive behaviour. This variation is the result of a complex system of interacting genes and environmental factors, which is poorly understood. We have studied the genetics of aggression in families of Golden Retriever dogs. Golden Retrievers are usually friendly pets. However, there are reports of very aggressive Golden Retrievers. A genetic background has been suggested for this aggression because the behaviour seems to occur more often in certain Golden Retriever families. The low genetic heterogeneity within dog breeds implies that only a limited number of the genes that influence a certain behavioural trait will be functionally polymorphic within a breed. This greatly facilitates the chances of finding such genes. We aimed to identify genetic variations underlying the variation in aggressive behaviour in Golden Retrievers. Correct phenotyping is the key to success of genetic studies. We initially phenotyped the dogs with a behavioural (aggression) test and a personal interview with the dog owner. It turned out to be difficult to elicit aggressive behaviour in the test. We therefore designed a mail questionnaire based on the canine behavioural assessment and research questionnaire (CBARQ; Hsu and Serpell 2003). Our results suggest that this questionnaire is a reliable and valid method for evaluating canine behavioural traits. We estimated heritabilities of several measures of aggression using the restricted maximum likelihood method. The measures of aggression were obtained either through owner opinions or by using the CBARQ. The aim of that study was to determine which of the aggression measures showed sufficient genetic variation to be useful as phenotypes for molecular genetic studies. The most reliable heritability estimates seemed to be those for simple dog owner impressions of human- and dog-directed aggression. In addition, several CBARQ derived measures showed clear genetic differences between the dogs. The heritability estimates were high, suggesting possible involvement of a major gene. Alterations in brain serotonin metabolism have been described in aggressive dogs. Therefore, four genes of the canine serotonergic system were cloned, characterised, and studied in Golden Retrievers. We evaluated the serotonin receptor genes 1A, 1B, and 2A and the serotonin transporter gene as candidates for aggression in Golden Retrievers. We performed mutation screens and an affecteds-only linkage analysis. We found no evidence for a major role of these candidate genes in aggressive behaviour in the families of Golden Retrievers. With the completion of the dog genome project, genome-wide linkage and association studies in dogs have become feasible. Such studies are currently in progress.