Semi-dominant alleles are those that are less potent in heterozygous form (one copy) than they are in homozygous form (two copies), but which are potent enough to show a difference in phenotype from the wild-type allele (no copy). Recessive alleles show no difference with only one copy versus zero copies, and fully dominant alleles look the same with one copy as with two.
A lethal allele is a variation of a gene that will eventually cause death, perhaps even a failure of conception or birth.
Such is the case with the most common allele which results in a natural bobtail. This gene has been documented in a number of breeds including: the Australian Shepherd, Austrian Pinscher, Australian Stumpy Tail Cattle Dog, Braque du Bourbonnais / Bourbonnais Pointer, Brazilian Terrier, Brittany Spaniel, Croatian Sheepdog, Danish Swedish Farmdog, Jack Russell Terrier, Karelian Bear Dog, Mudi, Polish Lowland Sheepdog, Pyrenean Shepherd, Braque Francais / Savoy Sheepdog, Schipperke, Spanish Water Dog, Swedish Vallhund, and the Pembroke Welsh Corgi. It has also been intentionally introduced into the Boxer to keep the look of a docked tail in countries that have outlawed docking.
Congenitally short-tailed dogs are present in many breeds; however, the causative mutation located in the T-box transcription factor T gene (C189G) had only been described in the bobtailed Pembroke Welsh Corgis. We investigated here the presence of the T gene mutation in 23 other breeds (360 dogs, including 156 natural short tailed) in which natural bobtailed dogs exist. In the 17 breeds in which the C189G mutation was observed, there was a perfect correlation between this mutation and the short-tail phenotype. However, 6 breeds did not carry the known substitution or any other mutations in the T gene coding regions. No dogs were found to be homozygous for the C189G mutation, suggesting that the homozygous condition is lethal. In order to study the effect of the T gene mutation on litter size, we compared the number of puppies born from short-tailed parents to that born from long-tailed parents. In the Swedish Vallhund breed, we observed a 29% decrease in the litter size when both parents were short tailed.
The Boston Terrier, English Bulldog, King Charles Spaniel, Minitaure Schnauzer, Parson Russel Terrier, and Rottweiler exhibit natural bobtails but do not carry the C189G mutation, and the following breeds have not been tested to confirm what mutation causes their bobtails: the French Bulldog, McNab, Miniature Fox Terrier, Old English Sheepdog, Rat Terrier, and Tenterfield Terrier.
Like Merle and one of the Hairless alleles, the C189G mutation is considered semi-dominant and lethal, but unlike those other two, there has not been scientific observation of negative health effects (other than the lack of a tail obviously) in dogs with only a single copy:
In contrast with homozygous, heterozygous bobtailed dogs have not been reported to manifest any other abnormalities (Indrebø et al. 2007).
While some breeders are claiming that the bobtail gene is also superior to the other known lethal semi-dominants in that it does not produce live homozygotes (“it only hurts eggs”), there are documented double bobtails that have been born with severe deformities:
Recently, Pembroke Welsh Corgi puppies with severe anatomical defects having the homozygous mutation have been characterized. These puppies lacked tails, manifested anorectal atresia with severe alterations in the posterior lumbar region and spine, and had a failure to thrive (Indrebø et al. 2007).
So beyond the ethical implications of killing embryos (out of sight, out of mind for most breeders), the double bobtail allele can produce profoundly crippled live births. The ethics of this are a bit more problematic. Although given that other scientific inquiries were unable to account for surviving double bobtails, it’s likely that the homozygous form is particularly lethal resulting in only a small percent of surviving puppies. Unlike some theories that these fatalities have little or no effect on litter size (the assumption being that the embryo would fail early enough for another non-homozygote to thrive in its place), the above referenced study did show a nearly 30% reduction in bobtail x bobtail litters. Sadly, they didn’t report on bobtail x tail fertility.
Without a healthy example of a double bobtail, the most concentrated we can breed to maximize the number of bobtail puppies is Carrier x Carrier, as documented in the diagram above. The lack of a healthy homozygote for bobtail gene means that this mutation will never be able to provide a completely bobtail strain that breeds true. There will always be a 33% undesirable overhead (dogs with tails) in any bobtail breeding plan. If we chose to consider the lethal homozygotes as well, there’s a 50% undesirable overhead in the effort to breed bobtail dogs with this gene. I think that’s a significant enough percent to question the ethics and efficacy of a genetic solution versus a surgical one.
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