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June 29, 2012

Dr. Bruce M. Cattanach

This article is not strictly part of the ‘Genetics Can Be Fun’ series, for which another update needs to be written, but it does concern bobtails and is therefore included here.  It was written for a Welsh Corgi  feature in Our Dogs and presents some new information on the bobtail.

Twenty or so years ago Peggy Gamble and Patsy Hewan asked me to investigate the inheritance of the bobtail in Pembroke Welsh Corgis.  They provided a dozen or so pedigrees with tail descriptions appended, and it was clear that the condition was indeed inherited; it seemed likely that the inheritance was that of a dominant gene.  This conclusion was supported by the finding that bobtail pups appeared in crosses of bobtail Corgis with other normal tailed breeds.  It was also thought that the double-dose, homozygote existed and could be recognised by a very short or absent tail, whereas the single-dose heterozygote had an evident but variably longer tail that typically was still very short.

There the matter lay until I decided that it would be interesting to see how difficult it might be to transfer a single gene from one breed to another and then regain recipient breed appearance and type.  Knowing something about the Pembroke bobtail and, even in 1990, recognising the likelihood of a docking ban, I opted to try introducing the Corgi bobtail into my own breed, the Boxer.  The results and associated investigations considerably helped the understanding of the bobtail.  Thus:

    1. The first Corgi x Boxer cross produced 7 pups, 5 of which had bobtails, indeed suggesting a dominant gene mode of inheritance.

    2. Subsequent breeding of the crossbreds back to Boxers over several generations confirmed this inheritance; in each generation approximately 50% of pups had bobtails.
    3. Tail length in these obligatory single-dose heterozygotes was found to vary from near-absent to about one-third normal length.  The majority were of docked Boxer length - about 2 inches.

    4. With the shorter tails, there invariably was a ‘fatty’ pad at the end which tended to droop, and the extreme tip comprised a fine filament that is sensitive to touch.  These characteristics of the bobtail allow a clear distinction from a docked tail.

    5. With the longer tails, the fatty pad was less obvious.

    6. With both tail lengths, there was variable evidence of minor kinking; the incidence of kinking appeared to be higher with the longer bobtails.

    7. The Norwegian Veterinary School has screened for spinal abnormalities in bobtail Corgis.  None were found.  And neither have any other abnormalities been found associated with the bobtail.

    8. Blood samples were collected from the Corgi - Boxer crosses over a number of generations and with the use of this material it was possible to screen for the gene responsible.  A first check aimed to see if the bobtail was caused by the T-box gene that causes a similar type of short tail in the mouse; and so it proved.  We had the gene at first attempt.  The Corgi bobtail has therefore been assigned to the canine T-box gene that lies on chromosome number 1; and the actual DNA mutation within this gene has also been identified.

    9. Using a gene test, we found that all screened bobtail Corgis that derived from bobtail x bobtail breeding were single-dose heterozygotes; none were double-dose homozygotes.  This provided the first evidence that the bobtail condition does not breed true.

    10. The Norwegian Veterinary School has reported two grossly abnormal Corgi pups at birth, one in each of two litters from a single set of bobtail parents, and indicated they were homozygous for the bobtail gene.  This is the only report of bobtail homozygotes that I am aware of in any bobtail breed.   Overall, therefore, it seems most probable that homozygous bobtail pups  generally die well before birth and are then naturally resorbed.  Consistent with this is a variable finding of marginally smaller average litter sizes with bobtail x bobtail breeding.

    11. The only expected consequences of bobtail x bobtail breeding is therefore a slight increase in the frequency of bobtails, from 50% to 66%, and possibly a minor reduction in litter size.

    12. With the use of the gene test it has also been possible to screen other breeds with similar short tails to see if these are also caused by the T-box mutation.  This work was done in Finish and French Veterinary Schools.  They found that 17 out of 23 short tail breeds studied that ranged from the Vallhund, through to the Australian Sheepdog, and to the Old English Sheepdog, had the same bobtail gene mutation.  This means that despite the diversity in size and shape of these 17 breeds, they must all be ancestrally-related.

    13. Interestingly, with only two exceptions, all these breeds are farm/cattle/ sheep/working dogs, and the two exceptions have this working dog ancestry. It would seem that the bobtail is virtually a defining characteristic of dogs used from ancient times to work livestock.


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