In dogs I follow closely the ideas of Bob Wehele of the world famous Elehew Pointer Kennel. His view is that if you get defective progeny then one of the parents must have been defective. Having said that he would not consider using a dog or bitch to breed unless they had made the grade.You can peruse his ideas for yourself and although Wing and Shot is his better known book Snakefoot, The making of a Champion is his most recent.
Whilst Bob Wehle's achievements are amazing not many people have the money to breed on the scale he did and very few people are comfortable with the large numbers that need to be "disposed" of in a program like this. IIRC he also kept a good number of distinct lines in his own kennel?
It is also impossible to argue that inbreeding should always be avoided as we simply wouldn't have the breeds today without it. Whether it is still necessary now is a different question altogether.
High levels of inbreeding (across a population of dogs, there will always be individual exceptons) can have the following effect. Dr John Armstrong performed a study on his breed (standard poodles) and saw that a SP with a 5 (?) generation Coefficient Of Inbreeding of 5% will on average live three years longer than one with a COI of 35% and a 10% increase in COI will result in a decrease in litter size of 7%. ( http://www.canine-genetics.com/pgbreed.htm ) Immunity and fertility also decrease with rising COI. From memory size and IQ also decrease fairly quickly with increased COI.
Having said that a dog s more than just a COI, just as it is more than a hipscore or colour i.e. it is just one piece in the puzzle. Put two poor dogs with low COI to each other and you will probably end up with a litter of poor dogs.
DesO'Neile wrote:
Probably the most closely monitored breeding programme in the UK is that of the bovine dairy/beef herd. It may surprise you to know that the most commomly used mating to fix traits is the mother son cross and I believe that genetically you can't get any closer than that.
It may be the most commonly used mating to fix "fixable" traits (there is nothing to say that all desirable gundog traits are fixable, the "perfect" wirehair coat being a good example, it needs to be heterozygous to be exhibited and is therefore NOT fixable) but it certainly isn't a practice that is used routinely generation after generation. I was only talking to one of our puppy owners a few weeks ago (he is a dairy farmer and has more than a passing interest in genetics) about this very subject and he out crosses to ensure fewer problems at calfing, a higher percentage of live calves and a higher milk yield. By the same token most meat and laying hens are crossbeeds, purely for commercial reasons i.e. the higher levels of fertility and production make a compelling commercial argument.
Over use of popular sires is just as damaging to the genetic diversity of a breed as linebreeding. _________________ Fi 1 year old Weimaraner
Grace Weimaraner. Gone but not forgotten.
Harley Weimaraner. Much loved and sorely missed.
Gracious me Annie, you're asking the best bog breeders in the world about things like co-efficiency, what's health and safety got to do about breeding dogs! Look we have the biggest dog show in the world, what more proof do you want we know everything.That's why hundreds of thousands of UK dogs are exported worldwide, almost as many as those registered by the AKC.Where would the Germans be if it wasn't for all those UK ones they have imported to put back the quality in German dogs!
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be if it wasn't for all those UK ones they have imported to put back the quality in German dogs! 
