SaintGirl, exactly. Environmental factors can cause HD or ED to be expressed. However, it cannot be clinically expressed if there was not an underlying genetic predisposition.
Part of the problem with the apparent inability to eradicate or strongly minimize CHD in dogs has to do with 2 things:
1) dogs who present as radiographically normal who are not genetically normal, and will produce dysplastic offspring, and
2) the failure to consistently have all puppies in litters screened and results released whether normal or abnormal.
I, along with others I greatly respect, feel that BREADTH in a pedigree (siblings of breeding parters, siblings of parents, and siblings of grand parents) are MOST important in managing to select dogs for breeding who are most likely to have/produce normal hips (and elbows). Generations straight back of normal dogs is also important, but it is the breadth of the normal dogs in the pedigree that carries the most weight.
This is one reason why the OFA's website is such a useful and wonderful tool for today's breeders. It is also a reason why breeders should try their very best to get ALL puppies in a litter screened, and have the courage and love for their breed to release ALL the results.
JMO as always.