Some comments (based on conversations with a very good sports medicine vet) -
Wobblers often doesn’t show up until a horse’s skeletal system has matured to the point where the cervical vertebrae are “pinching” or “squeezing” the spinal cord. Sometimes there are very subtle signs as young as 3, but it more common for the horse to be in the 7-8-9 y/o range before the symptoms are pronounced enough to warrant myleogram.
It also often doesn’t show up until the horse is being asked to carry itself in a more “compressed” frame as it goes up the levels in dressage.
As for it being “genetic”, yes, it is possible. But before assigning blame to the sire, or mare, or even to the breeder for continuing to breed related horses, consider this:
Foals have rather fragile necks. They can easily suffer a neck injury if yanked on while being led in halter/lead rope, or they “act out” and yank on it themselves (they are foals, after all!), or if they fall the wrong way, etc. Such neck injuries usually aren’t apparent at the time, but they can predispose the horse to developing wobblers down the road. (It’s why some breeders I know - esp. in Germany - don’t like to put their young foals in halters, and instead leave them loose at the dam’s side.)
And knowing that, I would agree with comments that a field neuro exam should always be part of a PPE, and, depending on the value of the horse, possibly even cervical radiographs - although it takes a good vet with a really good high end machine to get clear cervical pictures. It’s actually best done at a clinic or vet school with a “big machine”, but even then, you sometimes can’t tell without myleogram.
It sucks to buy a horse that ends up with wobblers. Someone I know bought a pretty high end 5 y/o from Europe some years ago. He was super fancy, with all the earmarks of being not only a very good FEI Y/H prospect, but also an UL prospect. By age 8, he was barely rideable, and was diagnosed with wobblers by myleogram after cervical radiographs showed clear indication of severe vertebrae interference. By then, his symptoms had gotten so bad, he was stumbling and “listing” at times (and yes, he was checked multiple times for EPM, and Lymes). The trainer was afraid he was going to fall on someone, and she and three vets recommended euthanasia, but the owner refused. She ended up moving the horse elsewhere and “retiring” him. Interestingly enough, after the horse was no longer being asked to work “round” and on the bit, his symptoms slowly abated, and she was able after some time to start riding him again, although he could not do much work in a dressage frame.