In recent years I have noticed an increase of wrong information floating around the internet especially on facebook groups about the “bobtail” in Chihuahuas. I am hoping to clear up a bit of the misinformation that is currently out there.
The natural bobtail gene is present in purebred Chihuahuas. In my time showing and breeding I have encountered in older show lines more than anywhere else. Before the revision of the Chihuahua Standard in 1972, the Bobtail Chihuahua was allowed to even show. No one even looked twice at them. Quoting from breeder, author “Tressa Thurmer’s” book “Pet Chihuahua” copyright 1962 ~
“Tail – moderately long, carried sickle either up or out, or in a loop over the back, with the tip just touching the back. (Never tucked under.) The hair on the tail in harmony with the coat of the body preferred furry in smooth coats. In long coats full and long as in a plume. A natural bob or tailless is permissible if so born.”
Their Chihuahua ancestors have been passing them down for at least 80 years, and most probably longer. Therefore, even the most responsible breeders, who exercise meticulously selective breeding programs are not immune to the possibility of having a bob-tail puppy.
The condition is known as "bob-tail" or "short tail" is caused by the Brachyury mutation. The inherited trait is autosomal dominant which means that dogs who carry one or two copies of the mutation will have a naturally short tail. Dogs with two copies of the normal non-mutated gene will have standard tail length. In homozygous state, the Brachyury mutation is lethal in utero. For this reason, breeding two dogs with the bob-tail gene generally result in somewhat reduced litter sizes.
Test to determine the number of copies of the recessive bob-tail/short tail allele a dog carries. Dogs can be DNA tested at ANY age.
BT/BT/ Lethal -The dog, carries two copy of the mutant gene. This condition is lethal in utero.
BT/n/CB - Short Tail - The dog carries one copy of the normal gene and one copy of the mutant gene. The dog has a natural short tail. Heterozygous dogs can pass the mutation to their offspring with a probability of 50%.
n/n or CC Normal Tail - The dog has two copies of the normal gene and will have a normal tail.
In Recent years we have DNA testing we can use to make sure we don’t produce the Lethal form of this mutation. Our kennel uses Embark for Breeders, they test for Bobtail in the Chihuahua breed along with many other diseases, mutations, and coat coloring. Below is an example of Embark DNA testing.
Like any mutation, we do not want to cull all the carriers of one allele, this would be a disaster for the breed as a whole. Currently, we have a high genetic diversity of the breed. If we started eliminating such a significant portion of the population from breeding, we would suffer from high inbreeding ratios, low mortality rates in newborns, low litters sizes and much more. Carefully planned to breed and testing our dogs for the mutation and only breeding a dog with one copy to a dog with two copies of a normal tail gene.
So why do Chihuahuas have this mutated gene? Well, no one knows that for sure but the concept of “purebred” is very new one, and the development of this breed via DNA can be traced 15,000 years to Asiatic Wolves and Village dogs in Europe. These dogs were not a carefully planned but bred with what was close, hence over thousands of years, there is a little of every breed in our Chihuahuas today.
Many speculate that the Chihuahua could have a connection with the Malta dog Kelb tal-But (Maltese pocket dog)There is a striking similarity between the Chihuahua and the Kelb tal-But in appearance. On a curious note it is depicted on postage stamps as a bobtail and with a standard long tail.