Genetics and the Dog: Selection for Particular Traits

By | February 24, 2010

There is some evidence that withers height and body weight are quite highly inherited and that they would tend to be related. We have data from GSD’s showing that sixty-day weight in the breed is about 45% heritable and thus vveights at other ages would be connected, while the genes which influence weight will also tend to influence height. In a breed seeking to increase height, progress should be reasonably rapid by the use of the taller animals in the breed. At the same time, body vveight would tend to be increased (whether one wanted this or not) and other related features would happen. Increased size vvould bring with it a more rapid growth rate and possibly increased risk of hip dysplasia. It would probably increase susceptibility of the long bones to diseases like panosteosis and it might lead to reduced hind angulation. There is also some evidence that increases in withers height are associated with greater litter size, at least up to a certain point. In breeds which are physically small, the associated low litter size has both economic and genetic drawbacks but there is minimal scope for increasing skeletal size beyond the odd centimetre or two. In general, most breeders are content to select to remain within a fairly narrow height range. Though breeds like the Great Dane have been selected towards a greater height and have faced serious bone difficulties on route to this questionable end. Perhaps the most important aspect of conformation is general balance which is connected with proportions of height to length. The length of body and length of back are quite distinct, though all too frequently confused. In most breeds, the tendency is towards a dog that is slightly longer then it is tall. Thus, the GSD permits a range of 10:9 through to 10.8.5. In the 1950’s the breed was made much longer with many animals exceeding the 10:8 range and this unfortunate feature has been prepetuated to the modern day by certain judges and by a small hard-core group of breeders who seem unprepared to accept that it is a flaw. There is ample evidence from the work of Humphrey and Warner at Fortunate Fields in the 1930’s that the 10:9 proportions made the best working shape for that breed and, at most, it should not exceed 10:8.8. This is substantiated by evidence from working police dogs vvhich rarely are too long or too heavy, though often quits large. Failure to get proportions right will affect the whole picture of the dog and extending the length of back can bring with it weakness and possible spinal troubles. On the other hand, excessively short backs can be associated with spinal compression and a variety of orthopaedic problems. Selection for short or long backs is again selection for a Polygenetic trait and is usually achieved by reducing the size of the spinal vertebrae or by reducing their number. Post mortern analyses are needed to cheek these matters but evidence from species such as the sheep, suggests that vertebrae number can fluctuate and are thus under genetic control.

Table1 Percentage of dogs with seven incisors (upper jaw)

Breed Number of dogs seen Percentage with seven incisors
Bulldog 71 39.4
Boxer 140 26.0
Bullmastiff 36 8.0
Pug 101 4.0
Pekingese 74 1.3
Mastiff 23 0.0

In some breeds head structure is emphasised to excess and one ought never to lose sight of the whole dog, a feature sometimes not always remembered by judges of such breeds as Bull Terriers or Boxers. In some breeds, selection for broad heads has occurred with some degree of success in terms of achieving what was sought, but it can often mean that broadening the muzzle affects dentition. The data in Table 1 show the percentage of dogs in certain breeds with additional incisors on the upper jaw and show this to be a marked feature of certain breeds. The data relate to a period twenty years ago but are doubtless true today in these breeds and others. New foundlands, selected for broad muzzles, have often got seven or eight incisors instead of the correct six. This may be a minor problem but it is still a departure from the ideal. Breeding for excessive hind angulation in the GSD, such as was seen here in the 1950’s and is currently lauded in the USA, will lead to poor movement, crabbing cow hocks and so on, though there is no evidence that it is associated with increased hip dysplasia. By the same token, altering the eye shape of the Chow has undoubtedly helped to increase the incidence of entropion. Far too many Chows have to be operated on for this defect and cosmetic surgery of this kind is hardly the kind of thing dog breeders ought to be encouraging by faulty breeding methods. Similar eye problems are seen in Bloodhounds and St. Bernards, while in some of the Collie breeds selection for particular head shape may have influenced the spread of certain eye diseases such as CEA. It is interesting to note that work in Holland threw up a situation in Shelties where a high percentage of eye defects was seen. One particular line was found in which no eve problems whatsoever were seen but the animals were of what might be termed a rather outdated type – the kind which might have been thrown out of the ring but which could have at least seen their vvay out! In general terms, it would be fair to say that exaggerations are to be avoided in any species or breed. Excessive length, hind angulation size or massive heads are likely to bring with them difficulties connected with the excess and, as such, ought to be frowned upon in the show ring. The same is equally true of extremes in the other direction. The dwarfing of breeds will not only be associated with a reduced litter size and hence selectional difficulties, but there are likely to be associated growth problems, and difficulties, such as cryptorchidism, are possibly going to increase. The old adage of the best to the best is based upon sound genetics and.most selection for conformation will be based upon breeding together dogs with similar virtues but not the same faults. Thus, deficiencies in shoulder placement will be hopefully corrected by using mates with ideal shoulder placement. In this connection, it is crucial to correct conformational faults by using mates that excel in the trait concerned rather than another extreme. Small size is not corrected by using oversized mates but by using correctly sized mates; better still, mates known to be producing correct size. It is important not to select for early maturity. In some breeds, judges have sought pups for the highest honours and selected not those pups which looked rightly immature but those which looked like mature adults in miniature. This policy is, in fact, selection for early maturity and has been encouraged by such things as the Junior Warrant: a very ill conceived concept best forgotten. A dog is a pup for twelve months and an adult for many years. We should seek to select dogs vvhich last a long time as adults, not those which blossom early and may be overblown far too soon. Early maturity in a conformational sense may be associated with growth patterns best avoided. The final plea must be for greater information on conformational aspects. If we are going to emphasise conformation as we do, it allows breed clubs to collect data on the subject in the form of breed surveys such as, are done on the Continent, but follovving these up with analyses of all the date so that we may finally start to fathom out the extent to which trails are inherited and hence the success rate we are likely to experience in selecting for them. Some thirty years ago Michael Fox a leading American authority on canine behaviour and genetics, produced a paper tabulating known and suspected genetic defects in the dog. His list ran to some sixty-five items at a time vvhen comparable human lists would have approached one thousand known genetic defects. In the past twenty years, more work has been undertaken on canine genetics and we have seen the mode of inheritance of several defects documented beyond all reasonable doubt. Nevertheless, the number of simple genetic defects in the dog which have been identified as, or likely to be, genetic in origin is still small compared to that in man. This does not mean that man has more problems or the dog fewer since sparsity of information generally reflects a lack of incentive (scientific or financial) to work on canine problems. Dogs are no less likely to suffer from genetic problems than man and in some instances, particular defects in the dog do reach astronomical levels within particular breeds. The popular press will,at intervals. publish articles which highlight particular problems in a single breed or in dogs in general in recent years, the New Scientist has critically appraised dog breeding in general while quite recently the Shooting Times had a critical attack on the German Shepherd and its columns have been full of letters highlighting defects in gundogs. If we, as dog breeders are going to combat such adverse publicity and, more particularly, combat the defects themselves, then we have to understand how and why such things occur and which methods exist to tackle the problems that beset us. This article will look at the issue of simple traits in the sense of simple inheritance.