Primary Lens Luxation is a gentically inherited disease in several breeds of dog, not just Jack Russells. Although shaking toys might indeed cause the lens to luxate the dog would have to be genetically affected for this to happen, ie a dog that does not carry the genes for it won't become affected. My breed is also one that can suffer from this but there is a DNA test that has been available since 2009 which means the parents can be tested before breeding and there is no need to breed any affected puppies. Sadly it is mostly pedigree breeds which test and the thousands of dogs like Jack Russells are never tested so people don't know whether the parents are clear, carriers or affected. The disease is also often late onset so dogs are bred from before they get the physical signs of the problem. Even if buying a pet from any of the affected breeds you should insist on the parents being DNA tested or hereditarily clear. It took us ten years for the research to come up with the gene yet many people still don't bother testing and pet owners and their dogs have to suffer the consequences. Same with Staffies, how many pet Staffies are bred without their parents being DNA tested - they have two conditions there are tests for yet how many people bother?