Bobtail
The bobtail phenotype is caused by a mutation in an extremely important gene residing in the T locus. The gene is therefore called the T gene (T stands for tail) or Brachyury. The product of the T gene, T protein, is responsible for the activation of some of the genes involved in the generation of the puppy’s muscles, bones, tendons, vessels and cartilage in the hindquarters.
The alleles that are responsible for normal development, the wild type allele, are named +. The mutated allele arises as a deletion (loss) of the entire coding part of the T gene and is named T. As the coding part of the gene is missing, this allele is not capable of generating any gene products.
In order for a puppy to complete a normal development of the hind part of the body, it must be in possession of double doses of the T protein; it must, in other words, be homozygous for the wild type allele (+/+). Embryos heterozygous for this mutation (T/+) are only capable of generating half of the needed T protein. They cannot complete the normal development of the outer hindquarters (the tail) and are born with a tail shorter than normal. Homozygous (T/T) cannot produce any functional T protein and therefore cannot activate the genes needed for generation of the hind part of the body. Such puppies will show severe defects in the hindquarters (which can be missing all together) and will normally die halfway into the embryonic stage whereupon the mother will reabsorb them. Sometimes however, puppies will be born with severe malfunctions like spina bifida, a missing anus, and holes in the body because of missing tissues and so on. These puppies are normally stillborn or will die shortly after birth.
The wild type and the mutated allele show a special dominance relationship called incomplete dominance. In this kind of transmission the heterozygous phenotype will appear as the intermediary between the two homozygous characters. This means that in a cross between a bobtailed dog and a dog with a normal tail around 50 % of the puppies will be born with a bobtail of various lengths. In a litter between two bobtailed dogs 50 % of the puppies will be born with a bobtail, 25 % of the puppies should have a normal tail and 25 % of the puppies should die or be born with severe defects.
It is not only the homozygous that show phenotypic changes in the embryonic stage, the heterozygous show some characteristics that differ from the wild type as well. As described above, they are not capable of complete development of the axis and therefore are born with a tail shorter than normal. They can furthermore show more or less pronounced abnormalities in the loin, pelvis and/or tail region: the nervous system can show abnormalities compared to the wild type, the pelvis vertebrae can show abnormalities like fusion, you can sometimes observe that the dog is crooked down its axis and the dog can show problems in the anal area. It must however be emphasized that it very rarely will be observed to a degree that causes the dog discomfort. The tail will often be thinner near the constriction of the tail and the terminal vertebra will often be smaller. This can be felt as a small knob at the tip of the tail. An odd character of the bobtail is that the dog often will produce too much skin near the constriction of the tail; this can be seen as a loose fold of skin.
The term bobtail, described above, means dogs with a little short tail. Puppies that are born without a tail are genetically a little different from the bobtailed dogs. In order for a dog to be born completely without a tail another factor, called t, is required. If t is expressed with T (T/t) it will result in an individual being born without a tail instead of a bobtail. The very odd thing about t is that individuals being either heterozygous (t/+) or homozygous (t/t) for this allele not will show any phenotype different from the wild type. This means that t doesn’t have a phenotype of its own, but will reinforce the phenotype that T is expressing.
Finally, to make things even more complicated, proof has been found that at least one other allele from another loci exists which is capable of normalising the phenotype of an otherwise bobtailed individual, meaning that it neutralizes the effect of T. Such individuals will appear with a normal tail, but are capable of transmitting bobtails to their offspring.
Below are some examples of test-crosses between dogs with different lengths of tail. Only the alleles T, +, and t are included.
