from Leon Whitney in How to Breed Dogs revised edition 1954
One of the most disappointing things that can happen to a dog breeder is to have what appears to be an almost perfect specimen born and raised, only in the last few months of growth to have it become ‘undershot’. There are puppies born which develop this misfit jaw characteristic in their first few weeks of growth, others which develop it at three or four months and others not until after five months. And it is not necessarily those which are most seriously affected which show it early. One of the worst examples of this protruding lower jaw I have seen was in a cocker Spaniel which up until teething time had a perfect fitting set of teeth; the lower incisors fit right behind the upper, when the mouth was closed. When she was seven months old her lower incisors protruded three-quarters of an inch.
Is this character inherited? Most certainly yes, but in what strange manner, no one has yet been able to say with certainty. And there is the opposite character in which the lower jaw is too short for the upper, known as ‘overshot’. There is as yet no definite measurement for us to say whether the trouble lies in the mandible being too short or the upper jaw has grown too far forward.
Such peculiarities might be expected to appear in crosses between different breeds, like a Collie mated to a Boston Terrier, but they appear within a pure breed. They are especially prone to appear in the Collie breed, although I have seen it in many breeds. This mode of inheritance is likewise not known.
Undershot Cocker Spaniels, in a closely bred strain, throws some light on the problem. Among my Cocker Spaniels there is not an undershot puppy or adult in the kennels. But every year a goodly number of undershot puppies appear. Therefore, one might reason that the character is recessive. But let us see. In the first place when they do appear, they do not necessarily appear in a twenty-five percent ratio. A very wonderful bitch named Charm, whose mouth was perfect, was mated to a dog name Red Brucie, whose mouth was also perfect. They produced four puppies every one of which was badly undershot, and one with a perfect mouth. Her name was Kathlyn. Kathlyn was bred to Champ, a son of Roderic. In all the puppies of Roderic, I have not had an undershot puppy, and he was bred to many bitches. But when Champ was mated to Kathryn on many occasions, there were always one or two undershot puppies. But their puppies were so fine that it paid to mate these dogs and destroy the undershot puppies. They had seven litters of which ten puppies were undershot. So here it would seem that the trait was recessive. But let us look further. Some undershot puppies have appeared from other parents. I mated a pair of these, which were not badly deformed and, of five puppies in a litter, not one was undershot. If undershot is a recessive, then all of these puppies should have been undershot.
Again, we have a case of character which runs in families, which seems to be inherited as a recessive, and yet does not behave that way consistently. There are all degrees of the defect. In fact, if we believe that dogs’ teeth to be a correct bite, should allow the lower incisors to slip behind the uppers, then an even bite of the front teeth is a little undershot, and possibly there has been so much selection for an even bite that dog breeders have unconsciously been breeding undershot dogs. If they are undershot a little, then a little addition to that little makes them appear badly undershot. And it is hard to draw the line.
An interesting study in a strain of long-haired Dachshunds was made by Gruenberg and Lea. Their dogs were so badly overshot that the canine teeth of the under jaw (mandible) occluded behind the upper teeth, instead of in front of them. The tooth size was reduced by the factor and the lower jaw appears to be shortened, and the upper jaw lengthened. This is seen in many breeds. I know a strain of Borzoi which was so badly affected that some of the puppies could not eat out of a pan normally. Gruenberg and Lea found, upon conducting some matings that this trait was inherited as a simple recessive.
The lower jaw or mandible sometimes is much too short. Phillips found this condition to be a recessive, with modifying factors, so that when the maximum expression of these factors occurs the jaw is so short as to cause the death of the dog.
- Gruenberg, H and Lea, AJ An Inherited Jaw Anomaly in Long-Haired Dachshunds Jour Genetics 39:285 1940
- Phillips J McI Pig Jaw in Cocker Spaniels Jour of Hered 36(6):177 1945