Breed-Defining Chondrodysplasia

Breed-defining chondrodysplasia, also called short-leggedness, is a favorable phenotype found in some breeds. The well-known features of this breed-defining trait are caused by the FGF4 retrogene insertion. Short-leggedness caused by the FGF4 insertion is a desirable trait, and it is caused by different mutation than the undesired dwarfism causing genetic disorder found in some breeds, such as the Norwegian Elkhound.

Our research team studied short-leggedness in almost 17 000 dogs as a part of studies on dog's genetic size. The chondrodysplasia causing FGF4 insertion was found in individuals in 40 breeds, and in 16 breeds all individuals were observed short legged. Clear examples of these visibly short legged breeds (all individuals have inherited the FGF4 insertion from both parents) were the Dachshund, the Swedish Vallhund, the Welsh Corgi, the Australian Terrier, the Dandie Dinmont Terrier, and the West Highland White Terrier. In addition to the short legged breeds verified in previous studies, our research team discovered some new genetically short legged breeds, such as the Norfolk Terrier, the Cesky Terrier and the Sealyham Terrier. Breeds or individual dogs that have inherited the FGF4 insertion from both of their parents were discovered to be in average 1.4 times smaller in height than normal legged dogs with the same weight. A dog that has a heterozygous FGF4 insertion, which means that it has inherited only one short leg causing allele, has a leg length that is something between long and short. 

In few breeds, such as the Havanese or the Miniature Portuguese Podengo, all different types of the FGF4 alleles are present in the breed; this allows variation of different leg lengths from short to medium or long. For instance, nine out of ten Havaneses, two thirds of Miniature Portuguese Podengos and every other Jack Russell Terrier is short legged, but also mid-sized or long-legged individuals are encountered in these breeds. In breeds that are diverse in their leg length, many different heights may be found in individuals that have the same weight. 

In tiny breeds, such as the Chihuahua or the Mi-Ki, the effect of the FGF4 gene may be hidden by some derived size markers and as results, the dog may have a long-legged phenotype even if the genotype is short-legged. Yet still, the FGF4 insertion is more common in smaller breeds and in fact, some size reducing markers, such as the derived GHR1 allele and the heterozygous GHR2 allele are more common in chondrodysplastic breeds. 

Even if the FGF4 insertion is more common in small breeds, also some larger breeds are found to be short legged. For instance, the Basset Hound (23 kg / 50 lbs., 36 cm / 14 “) is the largest breed that has a fixed, homozygous FGF4 insertion in all individuals. The Welsh Corgi Cardigan (14 kg / 31 lbs., 30 cm / 12 “) is the smallest breed in height in which some individuals carry the ancestral combination of size markers (the combination found in wolves). A similar ancestral combination is carried also by the Whippet that has an identical (14 kg / 31 lbs.) average weight to the Corgi. However, despite the athletic build of the 47 cm (18.5”) Whippet, it is clear that these two breeds see the world from two different heights because of the FGF4 insertion. 

Genetic size of the dog may be determined according to the size markers found in the MyDogDNA test panel. In addition, if the dog has the FGF4 insertion, the appropriate height value can be deducted from the size estimation, depending on whether the FGF4 insertion is inherited from one or both of the parents. The true size of the dog however is a complex sum of also many additional factors, such as the environment or other, yet unknown genetic markers.