A horse bred to be halter (conformation for the non horse folks) horse for the large muscles and heavily line bred to one Tb sire produced such a horrible inherited problem and something that they claim Impressive himself never showed signs of.
However isn't it interesting that in the history of Thoroughbreds to recent times that they don't have HYPP??? In my opinion, HYPP can't be blamed on an outcross to a different breed when that breed doesn't have it
Here is a quote from an article at UC Davis:
This genetic defect has been identified in descendents of the American Quarter Horse sire, Impressive. The original genetic defect causing HYPP was a natural mutation that occurred as part of the evolutionary process. The majority of such mutations, which are constantly occurring, are not compatible with survival. However, the genetic mutation causing HYPP produced a functional, yet altered, sodium ion channel. This gene mutation is not a product of inbreeding. The gene mutation causing HYPP inadvertently became widespread when breeders sought to produce horses with heavy musculature. To date, confirmed cases of HYPP have been restricted to descendants of this horse.
http://www.vgl.ucdavis.edu/services/hypp.php