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Date Posted: 16:27:50 08/27/09 Thu
Re(2): INBREEDING AND PEDIGREE DOG BREEDS - article conclusion
Posted on August 24, 2009 at 19:05:20 by Tracey
Anomalies
The dog has about 30,000 genes and there are about 400 known defects reported, many of them simple autosomal recessives or sex linked recessives and a few are dominant while some are polygenic. Around two dozen have been located by DNA testing and such defects can thus be rapidly eliminated from a population if testing is applied. For example PRA has a low incidence in the Irish setter and could be eliminated completely if all breeding stock were DNA assessed and those carrying the PRA gene culled from the breeding pool. Sometimes, as in copper toxicosis in the Bedlington terrier the gene is so widespread that a simple culling operation would be too drastic and a careful use of carriers to Normals undertaken to preserve other attributes.
Where DNA testing applies then breeding restrictions need only apply to the removal from breeding of the Nn and nn animals (NN being homozygous normal). In the case of other defects we must assess the nature of the defect and the stage at onset.
Much as one does not want pituitary dwarfs in the GSD it is obvious by about 7 weeks of age and thus represents economic loss to the breeder not the buyer. Such dogs can be given away or culled and if the concept of culling offends then such easily offended people should not be in animal breeding. All species including man have defects and always will because of mutations and we have to learn to live with the problem. It is not the most important feature of dog breeding. Late onset defects like some types of PRA are undesirable and breeding programmes need to be adjusted to identify carriers if this is not feasible (as yet) by DNA testing.
Polygenic traits like behavioural flaws, hip dysplasia, epilepsy, elbow dysplasia etc are more complex and where grading/scoring schemes etc are available then ALL breeding stock in appropriate breeds must be graded/scored and the breed clubs at general meeting must decide what is an acceptable level of grade/score and then the appropriate kennel club should impose regulations. Scientific advice should be taken from those scientists with experience and their ideas used in drawing up rules but this does not require breeding restrictions or limits on sires which achieve the criteria.
In a field like hip dysplasia a level of acceptability should be drawn up for appropriate breeds (not the same for each breed) and only dogs/bitches used that attain this level. Then progeny test data should be collected and if a sire (or dam) is producing poorly they should be removed from the breeding pool. No rules of this type yet apply in Britain but progeny averages are published and do lead to the reduced use of specific animals that had good results themselves but which did not live up to these as sires. Most breeders of experience have enough common sense to know that there is nothing to be gained breeding from a poor producer and they do not need some scientist telling them how to breed if the scientist(s) concerned have no experience of breeding dogs other than for vivisection purposes. The Zuchtwerts produced by the SV in Germany by Dr Reiner Beuing are more useful than blanket restrictions on use without such data being available.
Limiting sire usage
Although most publicity is given to abnormalities of a generally Mendelian nature the really important traits like behaviour, shape, construction etc are polygenic. Genetic theory states quite clearly that response to selection per year is given by the formula:
R = h 2 S
t
where R is the response per year, h 2 is the heritability of the trait, S is the selection differential and t is the generation interval.
As an example let us take a trait like wither height which in the GSD has a heritability of about 0.63 (63%) and let us assume that mean wither height( ignoring sex) is 64cm. Let us assume that the standard deviation of wither height is 2 cms. Let us further assume that generation interval ( the mean age of animals when their offspring are born) is 5 years. The selection differential is the superiority of parents relative to the mean of the population from which they come.
Let us assume that we are going to seek to make the breed larger in wither height. I am not saying that this is a logical objective but I am using it to show what restriction of progeny numbers can do. Let us assume that breeders want to use the best 10% of sires which, in this case would be the tallest animals. Then, using standard tables, these 10% will exceed the mean by 1.755 standard deviations which is (2 x 1.755) or 3.51 cms. In other words the best 10% would average 67.51 cms.
Progress through sires (we have ignored dams) will be on a per year basis:
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