The Dog’s Genetic Size – The Strong and Weak Size Markers

Understanding canine genetic size may be an important tool in the future, for instance, in planning of individual care, such as weight control. Previously we introduced the new size markers added to the MyDogDNA test panel. In this blog article, we will give a deeper insight into the effects of these markers.

The HMGA2 size marker has the strongest effect

The homozygous derived HMGA2 allele (two derived alleles) has the strongest size reducing effect of all markers. If the environmental factors, such as overweight, are not taken into consideration, no dog weighing above 12 kg (26 lbs.) has been identified as carrier for this marker. However, the dog size is not remarkably reduced if there is only one derived HMGA2 allele in the combination, with no other derived alleles. The largest breed in height, of which all dogs have the derived HMGA2 allele, is the Wire Fox Terrier (38cm, 8 kg / 15”, 18 lbs.), the heaviest being the Brazilian Terrier (10kg, 22 lbs.). The derived IGF1 allele along with the derived HMGA2 allele may be found only in small sized breeds, such as terriers.

The GHR2 size marker allows size variation within a breed

Generally speaking, the heterozygous derived alleles (only one derived allele) cannot usually be seen in the dog phenotype as reduced size. However, the derived heterozygous GHR2 allele is an exception. The derived GHR2 allele is found in only tiny dogs, and unlike other markers, the heterozygous GHR2 allele is more commonly encountered in smaller dogs than the homozygous GHR2 allele. The average size of dogs with heterozygous GHR2 allele is about 4 kg (9 lbs.), whereas the average size of dogs with homozygous GHR2 allele is up to 5.2 kg (11.5 lbs.). The GHR2 marker also brings out some size variation within the breed; whenever the derived GHR2 is present in the breed, so is the ancestral allele of GHR2. Every other dog that carries the derived GHR2 allele is from a short legged breed, which may even further reduce the height of the dog, but not necessarily the weight.

Two derived IGF1R alleles are expressed in the body size

The homozygous derived IGF1R allele can usually be found in small or medium sized dogs. A marker combination including only ancestral alleles, with the exception of derived IGF1R, has this far been encountered only in two breeds: Lagotto Romagnolo (44 cm, 13.5 kg / 17”, 30 lbs. ) and shorthaired Vizsla (61 cm, 25 kg / 24”, 55 lbs. ). However, the heterozygous IGF1R allele may be found in all size categories from the tiniest dogs up to American Akita, 40 kg (88 lbs.). Two derived IGF1R alleles are expressed in the dog phenotype as reduced size, however, the more there are derived alleles in other markers, the smaller the dog size is.

The derived IGF1 allele is common in the smallest individuals of some breeds

Not all derived alleles are exclusivity for small breeds, and in fact, some occasional weak size markers (IGF1, GHR1, STC2) may be found in all size groups. For instance, over 90% of the Rottweilers (46 kg, 62 cm / 24”, 101 lbs.) and nearly half of the Dogue de Bordeaux (63 cm, 48 kg / 25”, 105 lbs.) population carry a derived IGF1 allele. Because the derived alleles are also found in larger breeds, one can assume that larger dogs may carry some so called “giant markers” that hide the size reducing effect of the derived alleles. The Entlebucher Mountain dog (46 cm, 24.5 kg / 18”, 54 lbs), is the largest breed in weight with a completely fixed derived IGF1 allele, which possibly explains the smallest size among the Swiss Mountain dog breeds. In general, the derived IGF1 allele in an ancestral combination reduces the smallest body size found in each size category. For instance, the heterozygous IGF1 allele in an ancestral combination is found in dogs as small as 8 kg (18 lbs.), whereas the combination with homozygous derived IGF1 allele may be found in dogs as small as 6 kg (13 lbs.).

The effect of the STC2 size marker is moderate

The combination that has all ancestral alleles but homozygous derived allele in STC2 has a weight range from 10 kg to 50 kg (22 lbs. to 110 lbs.). The derived STC2 allele is believed to reduce the dog size, but the effect is weak when only one copy of derived allele is present. The derived STC2 alleles are fixed to only two breeds: Griffon Bruxellois (26 cm, 5 kg / 10”, 11 lbs.) and Phalene (28 cm, 3.5 kg / 11”, 8 lbs.). Another interesting note is the unpretentious nature of the derived STC2 allele; if there are more than three derived markers in the combination, the size reducing effect of STC2 becomes imperceptible, and does not reduce the dog size further. Another marker similar to the STC2 is the GHR1 marker. The derived homozygous GHR1 allele is found alone in otherwise ancestral combination in dogs weighing 10-55 kg (22-121 lbs.). The largest dog in height that has homozygous GHR1 allele is the Norwegian Lundehund (34 cm / 13”) and the heaviest the Swedish Vallhund (14 kg / 31 lbs.).



Rimbault, M.; Beale, H.; Schoenebeck, J.; Hoopes, B.; Allen, J.; Kilroy-Glynn, P.; Wayne, R.; Sutter, N. & Ostrander, E. 2013. Derived variants at six genes explain nearly half of size reduction in dog breeds. Genome research, 1985-1995.

Parker, H.; VonHoldt, B.; Quignon P.; Margulies E.; Shao S.; Mosher, D.; Spady, T.; Elkahloun, A.; Gargill, M.; Jones, P.; Maslen, C.; Acland, G.; Sutter N.; Kuroki, K.; Bustamante, C.; Wayne R.; Ostrander, E. 2009. An Expressed Fgf4 Retrogene is Associated with Breed-Defining Chondrodysplasia in Domestic Dogs. Science 325: 995-998.

Julia Bouirmane; Genetic variation influencing body size in purebred dogs