Thanks Bubba & Aspin. I've seen affected H/H horses but, of course, no one wants to cop to the seriousness of it all. So, an N/H horse can actually have a pretty severe case of HYPP? I was told 'it will always be very mild form', so was thinking as long as you kept N/H to N/N for breeding it wouldn't matter. If they can be so very miserable, it explains why you'd want to cut it out rather than to try to manage it.
I knew when purchasing my cremello to check status and only go for N/N, because I don't want to have to manage or breed around a deleterious condition.
I also only breed SCID clear to clear.