> edited cos ive just thought,if the dogs being tested are clear at that point does that mean its not a hereditory disease its a developing one.meaningif the dog is clear at that point and produces puppies the offspring will be ok??
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Some diseases are Congenital (present at birth) and some develop later, both can be hereditary.
Of course from clinical eye testing point of view the easiest and most satisfactory to deal with are the congenital ones.
The form of PRA our breed has that we now have a DNA test for was late onset, often showing no signs at all until after the dog had been bred from, sometimes as late as 6 years old, but in the past it showed up usually by about 3, so it shows that even with the same disease the time of onset can vary.
I expect those that showed it early were removed from the gene pool in the past so the only ones left were those that got it later and later.
Sadly with clinical eye testing you can only do your best and breed from dogs tested clear.
If they later turn out to be affected you have to backtrack with you breeding and inform the offspring's owners.
With luck as long as the partner was clear their offspring will only be healthy carriers, but of course can't be bred from unless a DNA test emerges for their status to be ascertained.
This is why DNA tests are so valuable you know what you have got, before any visible signs appear, and you can in theory safely use all dogs for breeding (though in practice afffected dogs would not be used as even to a clear partner al theri offsprign woudl be carriers, with no clears)without producing affected offspring (as long as mated to the right partner) this helps keep gene pools wider as you don't have to dump whole bloodlines that are connected to a problem.
The problem is to develop DNA tests you need affected dogs to use their DNA in research,a nd of course good breeders have striven over the years to minimise the risks and reduce the numbers of affected dogs produced as much as is humanly possible.