Brachycephaly

Brachycephaly is understood in the canine world as a shortening of the skull that has occurred because of the selective breeding of dogs. Typical brachycephalic breeds are the pug, bulldog, or boxer. These breeds are characterised by a short, upward pointing snout, a massive and rounded head and an underbite.

Brachycephaly is a quantitative trait and is therefore determined by multiple genes (the inheritance of the trait is referred to as multifactorial). It is reported that skull shape is regulated by at least five quantitative trait loci. Mapping in one of these regions revealed a missense mutation (c.1344C>A) in the BMP3 gene, which is nearly fixed in extreme brachycephalic breeds and is reported to be causal for this trait. However, genetic interactions with other loci affecting skull variability amplify or attenuate the effects of this mutation.

The BMP3 gene encodes bone morphogenetic protein 3, which plays a role in cranioskeletal development. Through its antagonistic action on other bone morphogenetic proteins and activins, it limits bone growth and thus provides the basis for the brachycephalic phenotype.

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References:

Schoenebeck, J.J., Hutchinson, S.A., Byers, A., Beale, H.C., Carrington, B., Faden, D.L., Rimbault, M., Decker, B., Kidd, J.M., Sood, R., Boyko, A.R., Fondon, J.W., Wayne, R.K., Bustamante, C.D., Ciruna, B., Ostrander, E.A. : Variation of BMP3 contributes to dog breed skull diversity. PLoS Genet 8:e1002849, 2012. Pubmed reference: 22876193