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Extreme phenotype: ways to handle it?

Gleroy

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Selection on exaggerated morphological features is probably one of the most important problem facing purebred dogs, one of the difficulties being to identify precisely how those morphological traits are affecting the health of the dogs. The recent study of Packer et al. (2015) however provides a very interesting example and suggestions on what could be done relative to the brachycephalic issue.

 

It indeed illustrates nicely how Brachycephalic Obstructive Airway Syndrome (BOAS), a chronic debilitating syndrome seen frequently in brachycephalic dogs, is connected, in a non-linear manner, with the shortening of muzzle length. According to this study BOAS occurred only in dogs whose muzzle length was less than half the cranial length. A majority of dogs which had this craniofacial ratio lower than 0.2 were found to be affected. The study found also that thick neck girth, obesity and neutering increased the risk for BOAS.

 

 

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With this craniofacial ratio, breed clubs now have a good indicator that could be used to reduce the incidence of this condition. It is clear that some good work could be made through the promotion of dogs with longer muzzle or even, inserting some condition on this ratio into the standard. As shown in the article, for some breeds, such as the Japanese Chin, it would probably be difficult to improve this ratio to needed values relative to BOAS but efforts have to be made any way, using crossbreeding if necessary. If stakeholders from the dog world do not move on this, it is clear that, at some point, some one else will, and it won’t necessarily be for the good of their breeds.

 

 

References:

Asher, L., Diesel, G., Summers, J. F., McGreevy, P. D., & Collins, L. M. (2009). Inherited defects in pedigree dogs. Part 1: Disorders related to breed standards. The Veterinary Journal, 182(3), 402-411.

Packer, R. M., Hendricks, A., Tivers, M. S., & Burn, C. C. (2015). Impact of Facial Conformation on Canine Health: Brachycephalic Obstructive Airway Syndrome. PloS one, 10(10), e0137496. http://www.plosone.org/article/metrics/info:doi/10.1371/journal.pone.0137496

 

Credit picture: I. Horvath



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I appreciate this post and discussion... thank you Gregoire!

I'd like to talk a little bit about Breed Standards.

 

Being a member of an AKC breed club, I have observed there to be powerful forces within the fancy, including members of Judge's Education Committees, Boards of Directors and well respected successful breeders who resist the temptation to make even minor Breed Standard revisions, let alone sweeping changes - the frequency of breed standards revisions falls many years apart. Education of all factions within the fancy as to the reasons for and benefits of altering a breed standard must be done - this is no small task and requires considerable communication efforts on the part of people charged with creating an accepted and officially adopted breed standard revision. I have noticed that no project of this nature is accomplished with any great speed in dog clubs. At this time, taking into account the international aspects of dog breeding and showing, taking the lead in revising a breed's standard becomes quite an undertaking involving many vested stakeholders.

 

Identification of breed standard issues which contribute to functionality including health and aesthetics which require written clarification is the starting point. I have observed widely varying opinions and some considerable diversity in individual's interpretations of words used in a breed standard to describe breed characteristics. Vague language used in breed standards to describe traits, intended or not, seems to allow for a drift into 'extremes'. Breeding for those extremes and judges awarding them is almost an inherent aspect of the sport of purebred dogs.

 

The consensus that must be reached as to exactly which points in a breed standard need to be clarified or changed to aid breeder's and judge's understanding of proper breed type is challenging. Changing the breed standard to increase muzzle length (head shape) by inserting specific language to describe the proportional relationship of muzzle to back skull could involve completely revising all aspects of head type description i.e. eyes shape, set, ears set come to mind; but really, the entire shape of the skull and fore-face would likely come into play as well as expression, and perhaps balance, carriage and neck, even body shape. I would wonder... where within an existing brachycephalic breed would dogs with longer muzzles come from? The examples available I'd imagine would be somewhat limited and clubs putting a mechanism in place for an overall evaluation of dogs by breed experts and/or veterinarians for breed specimens which typically are not shown at dog shows and are instead placed as pets for the reason they lack correct breed type could be a significant hurdle.  

 

Performance evaluations and testing, participation in working competitions do tend to keep breeds functionally honest to some degree.      

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I fully agree with you about the difficulty to change breed standards, I have only limited knowledge at it, but I heard that depending on breeds and their origins, the precision and place that is let for interpretation vary a lot, which would probably impact the efficiency of a change in standard.

 

Relatively to your last question on where could the dogs improving the dog morphology come from, as a geneticist, I would answer this would depend of the gene pool on the breed, as well as the genetic complexity of the trait. Just supposing the inheritance of your trait is polymorphic and you have a large population, it should be possible to select toward a better morphology, even if currently no dogs are showing the phenotype your require. In cattle, some breeds produce currently 10,000 litter per years, largely due to selection, while 50 years the production was less than half of this.

Yet the less simple in your trait inheritance, and the smallest in your population, the less chance you will have to make things move without relying on crossbreeding. In the case of skull shape, I think several candidate gene have been identified, which may help in selecting or introgressing some alleles of interrest in future, with development of adequate genetic tests...

 

Schoenebeck, J. J., & Ostrander, E. A. (2013). The genetics of canine skull shape variation. Genetics193(2), 317-325.

 

 

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