NATURAL BOBS
Are you marking and recording all your puppies that were born having natural bobbed tails with the "NB" designation? It's important!!!
From the RAT TALES/copyright 2000RTCA Pam Mills
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 is very important to keep track of this natural occurance because the gene that produces bobbed tails has hidden consequences.
The gene that causes brachury generally produces no obvious effects other thant 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 (Tt) 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 (Tt) individual usually dies without reproducing, it is only a concern when (inter)breeding 2 heterozygotes --individuals whose parents were 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--1/2 with long tails,1/2 with short tails. A bob-tailed female bred to a bob-tailed male, producing 9 puppies--6 with bob-tails & 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 2 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 apppear in a litter. Phenotypically (or physically) you can't identify a bobtail's specific gene type, but "the proof" may be 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 thes (T/T) pleitropic effects are seen as: missing appendages (metacarpal/metatarsals) or situations where the ribs, vertebrae (including spina 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 ofpituary 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 the registration papers and in pedigrees to know what you might be getting into. After all the "tell-tail-sign" might be missing.
I hope this article clears up any questions you might have on breeding two natural bob individuals.
Now here is some more info passed on from Sue N.
NBT is a dominant gene, which basically means three things: 1) You'll know who "carries" the gene because they will always visibly exhibit the trait, 2) Only one parent carrying the gene is required to pass the trait on to some of the offspring (in this case, because healthy NBTs carry only one dominant for the trait, with one recessive, non-NBT gene - not two dominants), 3) Those offspring exhibiting a full length tail do not carry the gene.
Number 3 is important only so that folks understand that just because a pup comes from a "NBT litter" or has a "NBT parent" doesn't mean that they carry the gene for NBT. Only those pups or dogs exhibiting a natural bob tail carry the gene for the trait.
~Sue
