The Big (and Small) Six


Domesticated dogs come in an extraordinary variety of shapes and sizes. From the tiniest teacup terrier to the largest Leonberger, the differences are astounding, and dogs have a greater diversity in body size than any other animal that walks on land. Humans have actively and intentionally been breeding dogs to conform to their needs and desires for hundreds of years. Still, it sometimes seems incredible that such an impressive diversity of sizes has been attained.

Recently, an international group of scientists discovered something that makes it even more incredible. In a study published in the September 2013 issue of Genome Research, they determined that approximately half of the weight differences seen across dog breeds can be explained by variations in and around only six genes. With support from the AKC Canine Health Foundation, the researchers were able to analyze the relevant sections of the genome in 500 dogs from more than 90 breeds and compare them to ancestral genotypes found in wild wolves, foxes, and coyotes.  Doing so has uncovered an enormous amount of data about how genetics influence size.

One of the most interesting findings of the research was that as the scientists looked at smaller and smaller breeds, they saw ever increasing numbers of genetic differences between those dogs and the ancestral canids. In some ways, that was not unexpected, as larger breed dogs are more similar in size to their wild cousins. However, it wasn’t that certain genes varied in dogs of specific sizes. Instead, smaller dogs had more variation in all of the alleles than their larger cousins. The notable exception to this rule was giant breed dogs, whose size was only poorly explained by the markers examined in this study. That suggests variations in the researched genes are capable of making dogs smaller but that it may take different areas of mutation, or possibly other factors, to make dogs even larger. 

The selective breeding of dogs to create clearly defined, and highly divergent, breed standards has made them an ideal system for understanding the role of genetics in determining height, weight, and other aspects of adult size. Dogs of each breed are similar in appearance and have significantly less genetic diversity than seen in other species. That lack of within-breed diversity makes it easier to link specific genes to their determined traits than it would be in a more chaotically variable species – such as humanity.

That said, the fact that dogs are uniquely suitable for this type of research doesn’t mean that discoveries based on the canine genome are only relevant to them. Studies such as this one can also provide insight into some of the size differences seen in humans and other species. Four of the genes identified by this research have already been linked to variations in human height, and the fifth has been shown to be associated with growth changes in mice.  These research findings may increase our understanding of growth-related health concerns in both dogs and humans.


Derived variants at six genes explain nearly half of size reduction in dog breeds (full-text available free online)

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