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Date Posted: 16:33:01 08/27/09 Thu
INBREEDING AND PEDIGREE DOG BREEDS - article
Posted on August 24, 2009 at 19:02:04 by Tracey
Malcolm B. Willis BSc (Dunelm, 1956) PhD (Edin. 1960)
Hon Assoc RCVS (1996)
Introduction
At the present time it is fashionable in some quarters to preach doom and gloom about pedigree dog breeds. Most of this is on the basis of inbreeding and much of it is circulating round the internet like some disease epidemic, or maybe a virus. Even editorials in the canine press have started asking questions. In some countries in Europe, notably The Netherlands and Germany, politicians are seeking to introduce rules about the breeding of dogs. I have little faith in politicians when they concentrate on politics and no faith whatsoever in such individuals passing judgement on biology, genetics and the dog on which they probably know nothing and are being advised by those with an axe to grind.
Although some types of dog (e.g. sight hounds) have existed in something like their present form for a possible 6,000 years, modern examples of Salukis, Afghan hounds or Greyhounds cannot trace their ancestry back to these early days. In the interim much intermingling will have occurred. Most modern breeds can trace their ancestry in pedigree terms back for some 100 to 125 years, some even less. The (English) Kennel Club was formed in 1873 and it is doubtful if reliable pedigrees exist before this date. On the basis that a generation in the dog is about 4.5 years we are talking of pedigrees that go back for some 22 to 28 generations.
All dogs descended from the wolf (probably various kinds) and began domestication some 10-12,000 years ago. It is popular to look at wolf behaviour and to conclude that dog behaviour has changed. Of course it has. Some behaviours seen in the wolf would have no purpose in a modern dog : burying food, for example, or vomiting up food for cubs. Some behaviours are not feasible because the conformation of the dog has been changed and this prevents some activities. Rightly or wrongly, we have tended to select dogs to retain their juvenile state and this has meant drop ears ( not as expressive as erect ones) while hairy faces and docked tails inhibit a dog's ability to convey messages to other dogs. That modern canine behaviour has changed from that of the wolf is not in itself a problem in the majority of cases. It could be in dogs that go feral but in most modern societies we are seeking to avoid that. The border collie herding sheep is following the hunting habit of the wolf but the exercise does not end up in a kill because man has brought about behavioural modification through selection. In contrast the fighting nature of some Pit Bull terriers is not the least bit wolf-like and represents the corrupting influence of man upon this tragic breed,
What one must be careful of is setting up the wolf as some kind of prototype to which dogs have to measure up and this is particularly true if we consider inbreeding. To discuss this we have to examine inbreeding.
The definition of Inbreeding
Inbreeding is popularly thought to be the mating of relatives. That is too general a definition. If we look at the pedigree of any individual of whatever species we find that ancestors double in each generation. We have two parents, four grandparents, 8 great grandparents etc. In generation 20 we actually have 1,048,576 acestors and twice that in the 21st generation. It must be obvious to anyone that, whatever species you examine, the time will come when there are more ancestors in a pedigree generation than there were individuals alive at that time. This can be readily demonstrated in any dog pedigree. It might be possible, in a numerically large breed to find nothing common within five generations but within ten there will be ancestors that appear several times and the further back one goes the more obvious this is. Take any Boxer pedigree and way back you will find Lustig v Dom. Take a BMD pedigree back for about 11 generations ( to the late 40's period) and you will find the Newfoundland dog Pluto v Erlengut. This will apply regardless of country of origin of the dog being examined. Though a few isolated lines might exist that are different they would be few and far between.
One cannot define inbreeding as simply mating relatives. The true definition is the mating of individuals more closely related than the average of the population from which they come. This means that a true definition of inbreeding could vary from breed to breed and from location (country) to location. However absolute values can be derived and assessed on that count.
Inbreeding is measured using Wright's Coefficient of Inbreeding which was first put forward in the 1920s. It can be expressed as a percentage e.g. 12.5% or as a proportion 0.125. It measures the increased homozygosity likely to occur in an individual. If you mate a Boxer to a Boxer you get Boxers which is no surprise because, of course, over the years many genes have become fixed in the breed. This means that all individuals carry the same combination of these genes.
For example, all BMD carry the non-merle version of the merle gene and are thus all mm at that locus. Similarly they are all a t a t and the greater majority are all BB though a very small number must be Bb and a very tiny fraction are bb. The fact that they are homozygous at these loci has no bearing whatsoever on the well-being of the dog and that will apply to a great many loci. Homozygosity is not a disease. Note also that although some inbreeding has been undertaken in the BMD the incidence of bb animals ( a colour problem not a disease) has not increased and most breeders have never seen one.
The consequences of Inbreeding
Whether we inbreed or not it would make no difference to genes that are fixed in all members of the breed but in non-fixed genes it would lead to an increase in homozygosity and a decrease in heterozygosity. If we have a gene that we will call N with alternatives N and n then we have three possibilities NN, Nn and nn. Inbreeding will move us towards the NN and nn versions at the expense of Nn. Most deliterious traits tend to be recessive. This is because if an undesirable feature is dominant a dog which has it shows it and thus is usually selected against, unless it is very late onset. Thus most abnormalities and defects tend to be recessive ( the nn equivalent) while normal animals are NN or Nn (called carriers). If inbreeding takes us towards NN and nn then the first thing we will see is an increase in deliterious defects. But many rare genes will be lost on inbreeding, it depends on the population. Moreover once identified the nn animals and some of the Nn ones can be discarded from a breeding programme. Inbreeding does not operate in isolation but coupled with selection and this is certainly true of the dog in respect of inherited defects..
Inbreeding BMD, for example, would increase the risk of hypomyelinogenesis (trembler) but only very slightly because it is already rare. In contrast it might not increase cancer risks (though they are not necessarily simple traits) because some 40% of the breed already die of cancer of one kind or another. However if your line does not carry a specific defect inbreeding will not create it. For example, our own line began with a dog of exemplary character who was the son of a "trembler carrier". This meant that our dog had a 50:50 chance that he carried the trembler allele and a 50:50 chance that he did not. Close breeding (> 25% inbreeding) within the line has shown that he did not carry the trembler allele and thus it is absent from our line. Do not forget that breeders, even if they inbreed, also follow selection so that we are not just talking about inbreeding but inbreeding with selection which is a different ball game.
What we do know is that inbreeding brings about something called inbreeding depression. This depends on a formula which is:
-2F dpq
This will look complex so let us examine this. The term F relates to the inbreeding coefficient. If the animal were a brother sister mating the F value would be 0.25 (25%) and if a half brother half/sister it would be 0.125. So 2F relates to twice the inbreeding coefficient in the population. The terms p and q relate to the frequencies in the population of the alternative genes. If, for example, half the genes in the breed were N and the other half were n then the p frequency (N) would be 0.5 and the q frequency (n) would also be 0.5. If only ten percent of the genes were n then p would equal 0.9 and q would equal 0.1. (A summation sign is needed because it must be done over all loci but is not given because my computer does not have it).
The term d relates to the degree of dominance. This is measured from the midpoint between the parents. Suppose wither height were a single gene trait and we mated a 66 cm animal to a 60 cm (having corrected for sex) then the mid point is 66+60/2 = 63. If this offspring from this mating averaged 63 cm there would be no dominance and d would be equal to 0.
If the offspring averaged 64 cm then this is 1cm above the midpoint and thus d = 1.
Clearly wither height is much more complex and the formula has the symbol to sum over all loci. However the term dpq is dependant upon there being some degree of dominance. If d = 0 then the term dpq is also equal to nothing and there would be no inbreeding depression. Similarly the pq part is higher when one is at intermediate rather than extreme values.
The belief that inbreeding always causes problems is too sweeping. If there is no dominance there is no depression for that trait. The critics of inbreeding rarely, if ever, tell you this.
Without going into too much mathematics it can be shown that in relatively highly heritable traits d tends to be small while in low heritability traits it can be large. Inbreeding depression as it is called is likely to be most obvious in what are called fitness traits ( fertility traits) and least obvious in traits like some aspects of conformation which tend to be quite heritable, often in excess of 0.40 (40%).
Inbreeding will not have much effect upon high heritability traits but could have in respect of low heritability traits. A breeder would rightly argue that maintaining fertility was desirable since without it the breeder cannot function. However what about prolificacy (litter size)? A pig breeder wants as large a litter as he can rear and thus inbreeding, if it reduced litter size, would be very damaging to a pig breeder. That is not true of a dog breeder.
A normal GSD litter is about 8 whereas a Pomeranian litter is closer to 2.
If one wants to pick a dog and a bitch to father the next generation then in the GSD case one is retaining 2 out of 8 or the best 25% whereas in the Pom one has to keep the whole litter and thus has no selection unless one produces a second litter. However a Pom breeder would need to have four litters to have the same selection rate as a GSD breeder and that will take longer in time to actually reach, increasing the generation interval. As a GSD breeder one wants a dog that is of sound character, is typical of the breed, free of major inherited problems and (if one is an exhibitor) that will win in the ring or win in the working side of the breed or both. If it is a companion animal then the winning aspect is irrelevant but the other features will apply. If one inbreeds and inbreeding had no effect upon high heritability traits but reduced litter size from 8 to 6 the litter would still be worth doing because a litter with one outstanding member would still be worth producing. A pig breeder wants uniformity and large numbers, a dog breeder ( and also a racehorse breeder) wants an exceptional animal and large numbers of mediocrity born is of some economic but little selectional merit. If we could each produce a litter of 6 with one world-champion and 5 average dogs that would be of more use to us and the breed than a litter of 8 average animals. I know few breeders who would be put off that idea.
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