Hereditary trait (not a disease) that determines the natural short tail through a mutation in the T gene (brachyury). The short tail is a morphological character; the health interest lies in its inheritance: T/T homozygotes are not viable (embryonic lethality), so mating two short-tailed dogs reduces litter size. It must not be confused with surgically amputated tail (docking).

The C189G mutation is present in approximately 17 of 23 short-tailed breeds studied; 6 short-tailed breeds do not carry it, so other genetic factors for short tail remain unidentified (Hytönen et al. 2009, PMID 18854372).

- Short or absent tail from birth (heterozygotes)
- No additional associated anomalies in heterozygotes
- Homozygotes: not viable, embryonic resorption
- Smaller litters when mating two carriers
- Absence of tail not due to amputation

Hereditary brachyuria in the dog was associated with the T gene (brachyury), a transcription factor key to the development of the notochord and the embryonic tail. The identified mutation, shared by many short-tailed breeds, produces a short tail in heterozygosity and embryonic lethality in homozygosity. The finding explained the reduced litters observed when mating two short-tailed dogs.

- The natural short tail is a dominant trait (T/t); T/T homozygotes are not viable.
- Do not mate two short-tailed dogs together: approximately 25 % of the litter is lost to embryonic lethality.
- Mate a short-tailed dog with a long-tailed (t/t) dog to avoid lethality, assuming that part of the offspring will have a long tail.
- Document the genotype (T/t vs t/t) to distinguish natural short tail from amputated tail.
- Inform buyers that the natural short tail carries this reproductive risk.

T brachyuria is a mild developmental anomaly in heterozygosity, but it should be distinguished from vertebral and tail defects caused by other factors (traumatic, non-genetic congenital defects). In breeds with traditional docking, the genotype must be confirmed to distinguish natural short tail from amputated tail. T/T homozygosity is embryonic lethality.

1. Hytönen MK et al. 2009, la mutación ancestral del gen T está presente en muchas, pero no todas, las razas de cola corta (PMID 18854372)
2. OMIA:000979 Cola corta (brachyury, gen T)