Are you marking and recording all your puppies that were born having naturally bobbed tails with the "NB" or "NBT" designation? It's IMPORTANT!
Natural bobbed tails (a natural shortening of the coccyx or tail bone) occurs in several
species and is seen in many different breeds of dogs, with our Rat Terrier's being one. *
It's very important to keep track of this natural occurrence because the gene that produces
bobbed tails has hidden consequences.
The gene that causes brachury generally produces no obvious effects other than the shortened
tail, when seen as a single recessive gene in the heterozygous (Tt/tt) condition. But, when
this gene for bobtails (brachury) appears in it's dominant homozygous (T/T) form, it then
becomes a lethal! (It works like the "Dominant W" or lethal white gene in homozygous individuals,
while it's recessive in heterozygous ones.) Since any homozygous (T/T) individual
usually dies without reproducing, it is only a concern when [inter]breeding two heterozygotes --
individuals whose parents were bobtailed or are bobtailed themselves, that this information
should definitely be considered when selecting breeding pairs, as it affects and can modify the
progeny ratios depending on the actual genotype of the crosses being made and in which
these T/T genotypes can be produced.
Example of inheritance: A long-tailed female is bred to a long-tailed male, producing 12
long-tailed puppies. A long-tailed female is bred to a bob-tailed male, producing 12
puppies--half with long-tails and half with short tails. A bob-tailed female bred to a bob-tailed male,
producing 9 puppies--6 with bob-tails and 3 with long tails. This example shows a phenotypical
ratio of 2:1 found in the viable offspring of the genotype T/+ versus the normal tailed +/+ individual
without known ancestors with this trait. Ordinarily, a cross between two bob-tailed individuals
is expected to produce a 1:2:1 genotypic ratio in the progeny, with their phenotypical ratios
being either 3:1 with complete dominance or 1:2:1 with intermediate or incomplete dominance.
Both long-tails and bob-tails will appear in a litter. Phenotypically (or physically) you can't
identify a bobtail's specific gene type, but "the proof" maybe in breeding. Interbreeding of T/t
and t/t genotypes, or a pair of bob-tailed (t/t) genotypes are expected to produce normal progeny
ratios of 3:1, often with a normal sized litter in which both T/t and/or t/t bobtailed genotypes
are produced. While a pair of T/t genotypes produces a litter with an expected 2:1 progeny ratio,
but with a markedly decreased number of offspring. These heterozygous recessive individuals
actually produce true 1:2:1 progeny ratios, and both (t/t) and (T/t) bobtailed genotypes are
viably produced, but approximately 1/4 of the offspring are homozygous lethal (T/T) gene types.
Since most individuals that are homozygous (T/T) for this gene die early during their embryonic
development, they may not be detected. Thus a smaller than normal litter may indicate
such a problem as breedings of these genotypes can often reduce the numbers of offspring
by 25%.
The homozygous (T/T) gene is also pleiotropic. Pleiotropic means a gene can also cause an
array of complex and interrelated abnormal developmental changes in organ systems,
and/or defects to both cartilage and structural developments. Most often these [T/T]
pleiotropic effects are seen as: missing appendages (metacarpal/metatarsals) or situations
where the ribs, vertebrae (including spinal bifida), and/or sternum are defective and are
associated with secondary notochord failures; resulting in the whole hindquarters being
malformed and/or frequently causing failure of the bones to knit properly together, and
which effectively can cause organs to be malformed. (Congenital hydrocephalus: ch allele - yet
another pleiotropic heterozygous lethal; with anomalies of sinus hairs, open eyelids, shape
of pituitary and ganglion Gasseri, intercerebral hemorrhages, ancephalus (missing cerebral
with cranial flat bone failures) are seen when (ch) allele is present, and while often seen in
conjunction with that of the T/T allele, are due to a different gene. )
Simply by avoiding breedings of two naturally bobtailed individuals helps to eliminate the
production of "tail-less" offspring, lethal T/T individuals and significantly
reduces stillbirths and the other birth defects this gene can produce. But it has to start with
you, the breeder. Since most Rat Terriers have their tails surgically docked, it's very important
for the genetic soundness of our breed that you keep accurate records, or it may not be
"known" if they are carriers of the gene responsible for this trait. As a breeder "you" are
the one responsible for the breeding and "docking". We all have to record this trait;
half tail, short tails, stumpy tails or no tail, by marking it "NB" on registration papers and
in pedigrees to know what you might be getting into. After all the "tell-tail sign" might be
missing.
*Toy Fox Terriers, Corgi's, Aussies and Manx cats are known to regularly exhibit this trait
in their breed, but it is also occasioned in Rotties and other breeds of canines. The t/t genotypes
are found to have a higher than normal incidence of complete brachury or tailessness /anchury.
Pleiotropism; H. Gruneberg / Genetics, 45, I-21, 1943. Genetics, M.W.
Farnsworth, St. Univ. of NY @ Buffalo
Pam Mills
Rattales@juno.com
Permission to cross post and use in it's entirety.
From the RAT TALES/copyright 2000RTCA Pam Mills
