Canine Diversity FAQ - Canine Genetics Diversity Articles

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Trace: » Population Genetics Made Easy! - Articles by J. Jeffrey Bragg » THE GENETIC TIDE: WILL IT LEAVE US HIGH AND DRY? » Canine Diversity: Frequently Asked Questions » table_of_dog_hobby_countries » Introduction: » Purebred Dog Breeds into the Twenty-First Century -- Achieving Genetic Health for Our Dogs » Purpose of This Site » DokuWiki » Canine Population Genetics in Practice: Principles for the Breeder » Random Genetic Drift -- The Breeder's Hidden Enemy
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====== Canine Diversity FAQ ======
> (Sue) we can cluster and organize these later, and then add the content! This will be a place where the less knowledgeable among us can make a contribution, by searching the archives and filling in blanks... which the SMEs can then polish and update.


===== What is a "breed" and how do breeds come about? =====


Basically a breed is a population of animals bred among themselves (//inter se//) within the protection of a pedigree barrier (meaning that ancestry of the breed comes primarily from a known founder group). Breeds are normally distinguished from one another in three ways, by ancestry, by typology, and by purpose. This means that not only does ancestry come from a particular source or group of founders, but that the breed is phenotypically distinguishable from other breeds and that it has a distinct character and purpose -- even if that purpose is just to be a companion animal.

Breeds come about in more than one way. Sometimes they develop in an isolated geographic region under natural and purpose selection; in that case they are called "landrace" breeds. Sometimes they are developed by a process of selection and breeding to serve a very particular end, as for example hunting a particular animal species in a particular way. Sometimes they are artificial creations brought into existence by crossbreeding existing breeds and selecting for particular traits of appearance and behaviour.

Due to these varied developmental modes, it is difficult to define precisely what is meant by a dog breed. Exceptions can be found to most generalisations in this area, but just about all breeds are characterised by //inter se// breeding behind a pedigree barrier, although the strictness of these principles may vary somewhat.

===== Wasn't inbreeding and selection required to develop all of today's dog breeds? =====

It is certainly true that today's dog breeds have all undergone a process of inbreeding and selection through which they have arrived at their present form and character. Indeed, this is a large part of the problem, since the breeds that we now have are all to a significant extent lacking in healthy genetic diversity (//depauperate//, to use the correct genetic term).

The irony of the situation is that the very fact that breeds have been developed in that way is now used as a pretext for continuing the process of inbreeding and strong artificial selection, even though these breeds are already "developed." The excuse is that we seek perfection, that "the breed must be improved" continually. There is no sound biological basis for proceeding in this way. The inbreeding/selection cycle, carried on indefinitely, results in depauperate breeds -- like the wild Cheetah, deprived of normal genetic diversity, vulnerable to environmental threats and disease. There is little evidence for the infinite perfectibility of dogs, or any other living creatures. Natural selection eventually produces optimal fitness, much of which depends upon genetic diversity. Inbreeding and selection, by destroying diversity, degrade fitness, thus producing a less perfect animal, no matter what may have been intended.

It is highly probable that our domestic dogs would be far healthier and hardier if, instead of some four hundred different breeds, we had only around forty dog breeds.  Much less inbreeding and selection would have been involved, much greater genetic diversity would remain in each breed. 


===== What is the difference between inbreeding and linebreeding? =====


To a geneticist there's no difference between inbreeding and linebreeding - it's all inbreeding.  That's because the mating of any relatives, even distant ones, contributes to the level of inbreeding within a line or breed. But in the dog world, inbreeding is commonly taken to mean the breeding of first-degree relatives (parent/offspring and brother/sister) and sometimes second-degree relatives, too (grandparent/grandchild, uncle/neice, aunt/nephew and half-brother/half-sister). Line-breeding is considered a "milder" form of inbreeding and usualy applies to the mating of third-degree relatives, such as first cousins, and beyond.


===== What is a coefficient of inbreeding (COI) and why should we care about it? =====


The coefficient of inbreeding or COI (Wright's Coefficient) is calculated from the known pedigree of an animal and represents **the probability that the two alleles (from the sire and the dam) of any given gene in an individual (or a mating) are identical by descent.** Usually expressed as a percentage, it may also be viewed as **the probable percentage of gene loci for this individual or mating that are homozygous by descent**.

The COI is significant because it gives us a quantitative measure of the inbreeding that occurs in any given mating, including the mating that produced the individual dog for which we have calculated a COI. The percentage figure gives us an easy way to compare matings without attempting to analyse long pedigrees. 

COI is customarily calculated to ten generations of pedigree, because to calculate to only four or five generations fails to give us the full picture. A ten-generation pedigree will often show quite significant levels of inbreeding even though no repetitions of individual ancestors might occur in the first four generations; this is called "background inbreeding." Simple pedigree analysis of necessity ignores background inbreeding, but the COI helps us take it into account.

Some typical levels of COI in simple inbreeding situations (without any background inbreeding) follow:

  * First cousin mating: 6.25%
  * Half-brother/half-sister, grandsire/granddaughter, and uncle/niece matings: 12.5%
  * Brother/sister (full sib) and father/daughter matings: 25%
  * Second generation full-sib mating: 37.5%
  * Third generation full-sib mating: 50%
  * Sixth generation full-sib mating: 73.4%
  * Second generation father/daughter mating: 37.5%
  * Third generation father/daughter mating: 43.75%

It is quite important for breeders to realise that even in the absence of near-pedigree inbreedings such as the above, multiple "linebreeding" repetitions of ancestors further back in the pedigree can result in levels of inbreeding equal to or greater than those obtained from brother/sister or father/daughter matings. 25% COI levels are common in many breeds and levels as high as 75% can occasionally be found in real-life pedigrees.


===== Are dog shows and other competitions good or bad for the welfare of dogs? =====

Natural selection is all about competition: competition for food, for territory and for the opportunity to reproduce. Natural selection is what has shaped every living animal and so competition can clearly be a force for good, moulding a species into a niche that gives it the very best chance of survival. 

In the case of working dogs, competition can play a very positive role. That's because working dogs often require considerable physical fitness (speed, agility, stamina). And so, as in nature, it is the fittest that tend to be chosen to produce the next generation.   

[more to come...]

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===== Is inbreeding really so detrimental to dogs? =====


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===== What is "genetic load"? =====


Genetic load is the relative difference between the most optimally fit genotype in a population and the average genotype of that population.

Genetic load is expressed in various ways. Most mammalian species have a number of lethal recessive genes (two of which will cause the death of the fœtus or the nestling), but that is not the full extent of genetic load. There are also many sublethal genes (such as those for heart defects, metabolic irregularities, or central nervous system disorders) that can seriously impact survival fitness. Finally there are non-lethal genes that nevertheless are suboptimal with respect to survival fitness, along with genes that reduce fertility and fecundity. The total of all these possibilities represents the genetic load of a given population.

Genetic load occurs through selection and mutation. It is meaningless to speak of "eliminating the genetic load" of a population as the only instance in which all members of a population would have an optimally fit genotype would be when all individuals shared the same identical genotype.

Management of genetic load in dog breeding should be oriented, not towards elimination of supposed genetic defects, but towards selection for fit genotypes and avoidance of reinforced lethal and sublethal genes.


===== What exactly do geneticists mean when they talk about "fitness"? =====


When geneticists or biologists speak of fitness they do not mean the kind of athletic conditioning that this word commonly brings to mind. Biological fitness is essentially the capability of a certain genotype to survive and reproduce successfully. Fitness measures the quantity of copies of genes (from a particular genotype) found in the next generation; it is a measure of reproductive success. Fitness can be and is measured and calculated by population geneticists.

From our point of view as dog breeders, anything that reduces fertility (successful matings), fecundity (number of progeny) or survivability of progeny must be regarded as "unfit."


===== Aren't genetic defects controlled adequately by screening and selection programs such as CERF and OFA? =====


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===== Do inbreeding experiments on insects and rodents tell us anything useful about dogs? =====


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===== If inbreeding is so bad, then how is it that some highly inbred populations of lab mice or dogs or wild animals manage to thrive? =====


Although highly inbred animal populations exist (mostly created by human intervention), it is questionable to say that any such populations "thrive." More accurately, they manage to survive for awhile. Inbred laboratory animals (mice, beagles, cavies) exist in protected environments largely free from environmental stress or challenge. Isolated inbred wild populations like the celebrated wolves of Isle Royale or the Cheetah are in reality struggling and vulnerable, ripe for extinction. Very soon (by nature's timetable) they will no longer exist; nature is in no hurry either to create or to extinguish animal populations.

That extremely inbred lab mice exist comfortably even in a protected environment is due largely to the fact that surviving strains have been successfully purged of many deleterious genes; their natural genetic load has been reduced by careful selection. Part of that selection process necessarily involves the fact that many bloodlines have been culled or discarded, or have simply failed to survive the process. The purebred dogs that now display dramatic genetic diseases for which screening programmes are being developed, are simply a demonstration of the risks of the inbreeding/selection process. Is it right that ninety-five dogs should die for each five survivors of a stringent inbreeding/culling programme? Is it worth that just to purge some part of the natural genetic load? Our companion animals are not lab mice and we should value their lives more than that.


===== How do registry structures and practices affect the genetic health of purebred dogs? =====


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===== What is a "closed stud book"? =====


The "stud book" is essentially the registry of individual animals within a breed, containing ancestry information and other data kept by a registry organisation such as the American Kennel Club. When a breed is first recognised and registered, a stud book is "opened" to what is called "foundation stock." These are the animals first registered as a distinct breed. Such animals are usually inspected for conformity to a stated type; there may be other requirements as to origins and particular traits. The stud book will usually remain open for a limited period of time and then be closed to further founder registrations by the registry organisation.

Thereafter animals of the breed in question will be registered if and only if they are descendants of the animals first registered, of the foundation stock. If no exceptions are allowed to this principle, then we have a "closed stud book" because no new genetic input will be permitted at any time. It should be noted that this practice is not the rule in many parts of the purebred livestock world, but it is generally the case with purebred dog breeds. A rigidly closed stud book has significant impact in the long run upon the genetic health of breeds and involves an inevitable steady loss of healthy genetic diversity over time.

===== Are the breeding strategies for promoting genetic health in a breed radically different for populous breeds than for rare breeds? =====


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===== How do the registration practices of A.K.C. and C.K.C. compare with those of registries for other species such as horses and cattle? =====


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===== What is meant by "form follows function"? =====


When we say "form follows function" in a dog breeding context we mean that the most efficient physique for a particular breed purpose will, over time, be produced simply by mating dogs that successfully fulfil that purpose, with no need for breeders to hold theories about the relative lengths and angular relationships of bones of the skeleton, or to breed according with such theories. The most successful examples of dogs able to fulfil their purpose will automatically be the ones that are best adapted physically to that purpose.

Novice breeders sometimes fall into the fallacy of believing that in order to breed a good working dog, it is necessary to practise canine engineering, attempting to construct a physical machine according to a plan or blueprint laid out in the breed standard. This amounts to an inversion of "form follows function" into "function should follow form." In practice, the most critical factors in working dog performance are usually mental, behavioural, and metabolic; the dog must have a mentality, behavioural traits, and energy metabolism adapted to breed purpose. These things are far more important than theories of "conformation," many of which are often far removed from the form that is actually most efficient for a given purpose.
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