Natural Bobtail

Natural bobtail (NBT) is a naturally occurring mutation in the T-box transcription factor T gene that results in a shortened tail. The mutation, a single nucleotide variant (c.189C>G), is inherited in an autosomal dominant fashion with both sexes equally affected. This mode of inheritance means that the presence of one copy of the variant will produce the bobbed tail phenotype in males and females. The length of the bobbed tail is variable and under the influence of other as-yet-undetermined genetic factors that cause some NBT individuals to have nearly full-length tails while others may have virtually no tail.

While heterozygous individuals (N/BT genotype, possessing one normal and one affected T gene) have shortened tails, data suggest that the homozygous condition (BT/BT genotype, possessing two copies of the affected gene) is lethal in utero in most cases. Breeding of two heterozygotes (a N/BT x N/BT mating) is predicted to produce 25% N/N puppies with normal length tails, 50% N/BT puppies with natural bobtail, and 25% homozygous affected BT/BT offspring. Most BT/BT genotype offspring are predicted to terminate in utero and therefore represent a potential 25% reduction in litter size.

One report in the veterinary literature identified two tailless Pembroke Welsh Corgi puppies with BT/BT genotype that survived to term but displayed severe developmental abnormalities incompatible with life, including anorectal atresia and spinal defects. Both puppies were from the same parents but different litters, and neither survived long after birth. This small sample size and lack of further reports suggests that it is rare for BT/BT puppies to reach full term.

Genetic testing is recommended to verify and validate the natural bobtail status of dogs, especially if docking is allowed, and to help with breeding pair selection to decrease the risk of reduced litter size and lethal deformities. Testing for natural bobtail assists owners and breeders in identifying dogs that have this trait.