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Long Term Inbreeding Within Breeds
Author Roy Robinson F.I.Biol.
(extract from "Genetics For Dog Breeders, 1990, Author Roy Robinson F.I.Biol.)
Inbreeding is commonly regarded as the mating of related individuals; especially closely related individuals, such as, brother to sister, father to daughter, mother to son. Half brother and sister, single and double first cousins matings are less close but still constitute inbreeding. The principle is that the mated individuals share one or more common ancestors. If these are sufficiently distant, this is not commonly regarded as inbreeding.If inbreeding is to be avoided, ideally, all ancestors must be different. However, this is impossible as a little calculation will reveal.
The number of ancestors per generation begins innocuously enough, two parents, four grandparents, eight G-G-parents and so on. The number is doubling in each generation.. Beyond 15 generations, the numbers will begin to increase very rapidly. It is not long before the number of ancestors will exceed the number of pedigree dogs bred in any generation, the number alive at a given time or the totality of dogs born to date.In reality, such numbers are impossibly large and in distant generations ancestors will be appearing more than once. Their presence constitutes inbreeding of a special sort which may be termed "long term inbreeding" or "background inbreeding." Such inbreeding is unavoidable. Unless a breed consists of sufficient numbers of breeding dogs and bitches per generation, the long term inbreeding can be a problem.
This type of inbreeding is largely independent of the actions of indivdual breeders.The rate of long term inbreeding can be calculated and is a function of the number of dogs and bitches in each generation. The smaller the number, the greater the rate of inbreeding. The theoretical rate has been derived and can be defined by a simple approximate formula. The formula is rate = 1/8M + 1/8F, where M is the number of dogs and F is the number of bitches per generation. Since long tern inbreeding is inevitable, is it possible to have a tolerable level? That is, a level at which the potential harmful effects of inbreeding are minimized? Two levels have indeed been proposed and these are rates of not more than five per cent and not more than one per cent.The five per cent has been proposed where it is possible to exercise tight control over the choice of breeding stock.
Every effort must be made to breed only from the most vigorous animals, with a good record of growth from a pup to maturity, excellent health and of proven fertility. If the above cannot be achieved, the one per cent should be chosen.In dogs, this will entail an notable interest in bitches which are on heat, a good record of conceptions and siring at least the average number of pups per litter for the breed. In bitches, this will entail regular heat periods, ability to conceive regularly, easy whelping to produce average litter size for the breed and the capacity to rear all of the pups with the minimum of assistance. Agreeable temperament should also be considered for both sexes. Selection for exhibition purposes should not be allowed to take precedence over the foregoing.
In genetic terms, the consequences of inbreeding is to increase the homozygosity of the genome. That is, the breed becomes progressively more homogeneous. The greater the rate of inbreeding the more rapid will this occur. This may be regarded as desirable This is how breeds have become established as their members converged to a common standard.However, increasing homogeneity can have an undesirable effect known as "inbreeding depression." This is because both good and bad traits are "fixed" with fine impartiality and the fixation may occur before all of the good points have been realized and the bad points have been eliminated. The greater the inbreeding, the quicker the fixation.Unfortunately, inbreeding depression is subtle and insidious and results in dogs which below par in growth, health, and breeding; just the opposite to the qualities listed above.
Only unremitting selection can counter the above tendency. By selection of the most vigorous animals, it is likely that the more heterogeneous will be chosen and this is helpful in countering the potential inbreeding depression.In practise, a compromise has to be found between the rate of inbreeding and the loss of genetic variation. The five per cent level should be regarded as the absolute minimum, with the one per cent as the more desirable.
A numerically small breed could be at risk in the long term but so could a numerically large breed if it is subdivided. On the other hand, provided that the subdivision is not absolute, a trickle of inter-group mating may be sufficient to maintain the well-being of the breed.The situation is more serious for those breeds with small number of breeding animals. Several methods may be proposed for mitigate the situation. One would be to popularize the breed to recruit new adherents. Another would be to encourage members to make their working stud dogs freely available. No stud dog to be the sire of more than a stated percentage of matings. Ideally, there should be as many breeding dogs as bitches and no bitch to be mated more than once to the same stud.To actively discourage matings closer than a given familial relationship. For instance, no common ancestors until the third or fourth generation as far as practical. Lastly, stud dogs to be imported from abroad although the pedigrees of these would have to be minutely scrutinized to ensure that these are as unrelated as possible. If all else fails and the breed is declining, an outcross to a breed of similar conformation will be necessary.
An investigation has been undertaken to estimate the amount of long term inbreeding in breeds with limited number of breeding dogs and bitches. For a large inter-breeding population, the amount of inbreeding is negligible. However, in a small population, such as a recently imported breed or a relatively unpopular breed, the amount may not be negligible. This will occur even if close inbreeding is disallowed in each generation. This policy will slow the inbreedinq but not prevent it.
The level of the rate of inbreeding which may be regarded as dangerously high in the long term is a matter of opinion. However, conversationalists have settled on a rate which exceeds one per cent per generation as too high for comfort. It is felt that this is the level to which all dog breeds should aspire. This level is found when the effective breeding population per generation falls below 50. This is equivalent to 25 dogs and 25 bitches, mating at random and contributing equally to the next generation.The rate of inbreeding per generation is cumulative and another measure of inbreeding is the number of generations for the genetic variation within the breed to be halved. If the rate of inbreeding is to high, both harmless and harmful polygenes may be fixed before selection has a chance to eliminate or at least reduced the latter to negligible proportions.
For an effective population of 50 individuals, the number of generations is in the region of 80, quite a lengthy period from a breeder's viewpoint but not for a breed. There may be scope for the period to be extended provided vigorous breeding stock are selected;Ideally, each sex should contribute equally to the next generation. Presumably the number of dogs will always be less than the number of bltches but the sex ratio should not depart to much from a 1:1 ratio. This seems to be the situation for the breeds examined.
Rigorous selection of vigorous and fully healthy breeding animals is advisable at all times. However, this dictum will particularly apply to those breeds where the rate of inbreeding exceeds one per cent. Breeders of such breeds would be strongly advised to adopt every available means of increasing the numbers of breeding stock or to use existing stock to best advantage. A number of suggestions in this respect have been outlined previously.
A separate analysis examined if one or more stud dogs sired a high proportion of litters. No evidence was found for the possibility in any of the breeds. A reason could be that the number of breeding animals in any generation was too few for a single stud to be used extensively.Also, the majority of breeders of numerically small breeds seem to be aware of the danger of allowing a single stud dog to feature too prominently in breeding.Copyright(c) 1996 Roy Robinson 1996