Breeding Out Faults

By Steven A. Robinson
Of 
Darkstarr Rottweilers

We have dealt with several different lines through the years and have found that with each generation, identifying the contributing factors that allow certain faults to manifest themselves, is an important step. This is especially true with polygenetic faults (faults that require multiple genetic traits in order to manifest itself). One example is bad bites. The alignment of the jaw is not dependent upon one gene but rather multiple genes. The length of the lower jaw is controlled by one set and the length of the upper yet another. This is why so many of established breeders do not like to breed together two vastly different head types. Even though the two dogs have scissors bites, they know the odds of the top and bottom jaws matching up in the pups is less likely. In fact, if given a choice between an out of proportioned head with a scissors bite and a well proportioned head with an even bite, I would prefer my odds with the even bite. It represents less of a genetic hill to climb. Another seeming polygenetic example is Entropian and Ectropian. Generally, a short eye lid combined with a small slanting eye (almond) will contribute to Entropian or Ectropian. Faults that appear to be a simple dominate are easier to get rid of because if the dog has it in his genotype, it will appear in his phenotype. You just simply do not use those dogs, and the problem and this fault will be gone. If the fault acts like a simple recessive, then your options are more complicated. If one of the parents has it in his/her genotype but not in its phenotype, your odds are 50% of the offspring will be carriers, 50% will not be carriers and none will display it in their phenotype. If both parents are carriers but do not display it in their phenotype , your odds are 50% of the offspring will be carriers, 25% will not be carriers and 25% will display the fault in their phenotype. If one parent displays it in their phenotype and the other is not a carrier, none of the offspring would display the fault but 100% would be carriers. If one of the parents displays the fault and the other is just a carrier, 50% of the offspring would be carriers and 50% would display it in their phenotype. 

    As a matter of practice, I would not use dogs who display the fault and use the mathematical probability of the above scenarios to identify those who are carriers. Once identified, then assess whether the other redeeming qualities each dog has is worth dealing with the fault. If so, as mentioned above, half of his/her offspring will be carriers. Unfortunately, most faults are not a simple dominate/recessive.

**Not for reprint without authors consent**