Testing Summary
The brachycephaly trait test reliably determines if a dog has one of the following genotypes:
| BR/BR | This dog carries two copies of the BR Allele which is found in dogs with medium to long muzzles. However, the actual muzzle length of the dog is a result of a combination of factors including multiple variants in other genes. This dog will pass one copy of BR to 100% of its offspring and can produce dogs with medium to long muzzles. Interpretation: Likely medium to long muzzle |
| BR/br | This dog carries one copy of the BR allele and one copy of br, the variant responsible for brachycephaly in some breeds. However, the actual muzzle length of the dog is a result of a combination of factors including multiple variants in other genes. This dog will pass the BR allele to 50% of its offspring and the br allele to 50% of its offspring, which can produce dogs with short muzzles if inherited with a second copy of the br allele. Interpretation: Likely medium to long muzzle (short muzzle Carrier) |
| br/br | This dog carries two copies of the br allele, which is associated with short muzzle length in some breeds of dog. This dog will pass br to 100% of its offspring and can produce dogs with short muzzles if bred to another dog who carries at least one copy of br. Interpretation: Likely short muzzle |
Detailed Summary
The skull shape of dogs varies significantly across breeds. This variation was originally introduced by selective breeding as part of early dog domestication but has continued in many modern breeds primarily through artificial selection of desired traits. Brachycephaly or “shortened head” refers to the short nose and face (muzzle) of dogs like Pugs, Boxers, and Bulldogs, among others. Although the overall shape and size of the canine skull is the result of multiple genetic factors, inheriting two copies of a Mutation in the BMP3 gene is associated with a short muzzle. BMP3 is thought to be essential for normal craniofacial development and mutations in this gene have been associated with abnormal craniofacial structures. Two copies of the mutation in BMP3 are nearly fixed in many extreme brachycephalic breeds including the Pug, Pekingese, French Bulldog, Brussels Griffon, and Boston Terrier. However, not all extreme brachycephalic breeds carry two copies of this mutation. Other genetic factors such as mutations in SMOC2 are thought to contribute to brachycephaly in other breeds.
Testing Tips
The brachycephaly trait test evaluates the BMP3 gene for the presence of the “br” variant that is associated with a short muzzle in some breeds of dog. Dogs that inherit one or no copies of the BMP3 Mutation (BR/BR or BR/br) are likely to have muzzles that are medium to long in length compared to dogs that inherit two copies and are more likely to have short muzzles. However, other genetic factors are known to contribute to canine head shape. Therefore, not all dogs with brachycephaly will be carriers of the “br” variant.
There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.
References
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Marchant TW, Johnson EJ, McTeir L, Johnson CI, Gow A, Liuti T, Kuehn D, Svenson K, Bermingham ML, Drögemüller M, Nussbaumer M, Davey MG, Argyle DJ, Powell RM, Guilherme S, Lang J, Ter Haar G, Leeb T, Schwarz T, Mellanby RJ, Clements DN, Schoenebeck JJ. Canine brachycephaly is associated with a retrotransposon-mediated missplicing of SMOC2. Curr Biol. 2017 Jun 5;27(11):1573-1584.
[PubMed: 28552356]
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Schoenebeck JJ, Hutchinson SA, Byers A, Beale HC, Carrington B, Faden DL, Rimbault M, Decker B, Kidd JM, Sood R, Boyko AR, Fondon JW 3rd, Wayne RK, Bustamante CD, Ciruna B, Ostrander EA. Variation of BMP3 contributes to dog breed skull diversity. PLoS Genet. 2012;8(8):e1002849.
[PubMed: 22876193]
