Inbreeding is NOT the correct word, line breeding is the correct term.
HYPP was a genetic mutation. It was not CAUSED by anything that is known. It was a genetic mutation in IMPRESSIVE. That is why it traces back to him. It began with him. The fact that his lines were so widely bred is the reason it spread. The fact that it took YEARS to trace it back to him is why it was so widely spread and the fact that it took many MANY more years for people to stop PURPOSELY breeding horses that carried the gene because it ended with desirable results in the halter arena is why it spread. There are OVER 100,000 horses with Impressive in their blood today.
Sorr about the rant but I'm passionate about setting the record straight when it comes to that horse. I hate that he is remembered only for the bad.