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Sunday, April 17, 2011

Aortic Stenosis in Boxer Dogs: Genetics and Inheritance

AortaImage via WikipediaThis is the second part of a series of articles on aortic stenosis in Boxers. View part one, Physiology. Further articles will be posted shortly.

Aortic Stenosis is a common heart condition in Boxers, caused by a ridge or narrowing on the wall of the aorta, at or just below the aortic valve. Aortic Stenosis is considered to be a genetic, inherited disease in dogs, and research on Newfoundlands and Boxers has supported this theory.

Originally the Newfoundland research revealed a dominant mode of inheritance, with variable penetrance. Further studies, however, have confused the issue, especially crosses to other breeds where the results did not meet the expectations of a dominant mode of inheritance. Many believe the disease is polygenic, involving multiple genes; a likely scenario is one "major" gene with additional genes that influence, trigger, or modify expression of the disease.

As an example of how this inheritance might work, consider coat colors in Boxers. Although one gene is associated with the brindle color, other genes effect the amount of brindling that a dog receives. Brindle Boxers range from having very few stripes -- what might be a Grade 1, mildly affected AS dog -- to having so many stripes they are mostly black -- a Grade VI, severely affected AS dog.

Data collected through the UK Breed Council control scheme for AS during the 1990s and early 2000s showed that parental grade is related to progeny grade. While dogs who test clear for AS may still occasionally produce an affected puppy, the degree of AS in the affected progeny is generally mild to moderate. The higher the grade of the parents, the higher the grade of the worst-affected progeny. This means that even though we do not know the precise genetics behind AS, we can decrease the incidence of the disease in the breed, especially at levels that would affect length and quality of life, by screening and removing the clearly affected dogs from breeding. This process has worked in the UK, where the breed went from being over-represented in severe cases of AS referred to vet schools to having only a very few cases referred.

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