Short tail (Bobtail, brachyury)

Gene: TBXT

Transmission: Autosomal, dominant

For an autosomal dominant genetic disease, an animal must have at least one copy of the mutation in question to be at risk of developing the disease.  Animals with two copies of the mutation generally have more severe symptoms and an earlier onset of the disease than animals with just one copy of the mutation.  One or both of the parents of an animal with the mutation has one or two copies of the mutation.  Animals that have one or two copies of the mutation can pass the mutation on to future generations.

Mutation: Substitution, TBXT gene; c.189 C>G, p.(Ile63Met), exon1, chr.1

Medical system: Skeletal

Breeds: Australian Shepherd, Australian Stumpy Tailed Cattle Dog, Austrian Pinscher, Bourbonnais Pointer, Brazilian Terrier, Brittany, Croatian Sheepdog, Danish Swedish Farmdog, Jack Russell Terrier, Karelian Bear Dog, Miniature American Shepherd, Miniature Australian Shepherd, Mudi, Parson Russell Terrier, Pembroke Welsh Corgi, Polish Lowland Sheepdog, Pyrenean Shepherd, Russell Terrier, Savoy Sheepdog, Schipperke, Spanish Water Dog, Swedish Vallhund

Age of onset of symptoms: From birth

A natural short-tailed phenotype, known as bobtail, is observed in numerous dog breeds, and has been used as an alternative to tail docking.  DNA studies in the Pembroke Welsh Corgi identified a mutation within the TBXT gene as being responsible for the bobtailed phenotype in this breed.  The TBXT gene encodes a protein involved in the embryonic development of the mesoderm that is important in determining the number of vertebrae formed in the posterior part of the developing embryo, in other words, in determining tail length.  The mutation is dominant and can have a variable penetration such that the tail is absent (anury) or merely shortened (brachyury).  Further DNA studies showed that the same mutation within the TBXT gene was also responsible for the bobtailed phenotype in numerous additional dogs, particularly in sheepdog and hunting breeds.  A double mutant animal was not seen, suggesting that the double mutant embryo is not viable.  Therefore, breeding two bobtail dogs together would decrease the size of the resulting litter.   For the Boston Terrier, English Bulldog, King Charles Spaniel, Miniature Schnauzer, Parson Russel Terrier and Rottweiler, the TBXT gene mutation does not account for bobtail phenotype observed in these breeds.  This suggests that there are yet to be identified mutations in additional genes that can be responsible for the short tail phenotype.

Mutations in the feline TBXT gene are similarly responsible for the short tail phenotype seen in several cat breeds such as the Manx.

References:

OMIA link: [0975-9685]

Hytönen MK, Grall A, Hédan B, et al. (2009) Ancestral T-Box mutation is present in many, but not all, short-tailed dog breeds.  Journal of Heredity 100(2):236-240 [pubmed/18854372]

Haworth K, Putt W, Cattanach B, et al. (2001) Canine homolog of the T-box transcription factor T; failure of the protein to bind to its DNA target leads to a short-tail phenotype.  Mammalian Genome 12:212-218.  [pubmed/11252170]