H/H foals can’t as of 2007.
Here is the blurb from the AQHA rulebook
" Effective with foals born on or after January 1, 2007, all descendants of the of the stallion Impressive, AQHA registration number 0767246, shall be required to be parentage verified and HYPP tested, subject to the conditions in ©(2) above. Any foal testing homozygous positive for HYPP (H/H) will not be eligible for registration with AQHA."
So, only horses homozygous for HYPP can’t be registered.
This is another blurb right off the AQHA site:
For some time now, you have probably heard, or had first-hand knowledge, of the condition known as hyperkalemic periodic paralysis (HYPP). This condition is characterized by intermittent episodes of muscle tremors (shaking or trembling, weaknesses and/or collapse).
At the 1996 AQHA Convention in Seattle, Washington, the AQHA Board of Directors approved some rules recommended by the AQHA Stud Book and Registration Committee and approved by the Board of Directors. Among the changes was a rule requiring disclosure of HYPP status on the registration certificates of foals born on or after January 1, 1998, which descend from any bloodline determined to carry the HYPP gene.
Beginning with the 1997 AQHA Official Handbook, HYPP is in rule 205 among conditions commonly considered undesirable traits or genetic defects, such as parrot mouth and cryptorchidism. These conditions do not prevent a horse from being used as breeding stock or from participating in AQHA-approved events, subject to rules of the individual event.
Beginning with 1998 foals, the rule requires the following notification to be placed on the registration certificates of foals descending from any bloodline determined to carry the HYPP gene:
“This horse has an ancestor known to carry HYPP, designated under AQHA rules as a genetic defect, AQHA recommends testing to confirm presence or absence of this gene.”
Facts about HYPP have been gained through research projects funded in part by AQHA, through the University of California, Davis and the University of Pennsylvania. The first report, from Drs. Sharon Spier and Gary Carlson of U.C. Davis, was delivered to AQHA in the summer 1992, and published in-full in the September 1992 issue of The Quarter Horse Journal. As additional information has been made available, AQHA has promptly published it. I invite you to refer to [the Publications] page for a list of AQHA publications and others which contain information about HYPP.
So…the AQHA basically says that they have had plenty of info on HYPP available since 1992, they thought about some rule changes in 1996 to be effective in 1998 that disclosing the defect became mandatory. At that point there was still no real incentive to not breed for it since all that happened was that your horse was positive for it now got stamped on his papers. (That they more or less think it’s OK to breed horses with HYPP, Cryptorchidism and parrot mouth too also boggles the mind but that is a whole other tangent.) As of 2007 you can no longer register a horse that is H/H (which is likely to be symptomatic) but there still is no real incentive to breed it out of the gene pool entirely since having N/H’s are fine. So…in 15 years you now can’t register a horse that is H/H. I don’t really see any plan to eliminate the disease entirely.
As noted earlier, the HYPP thing isn’t really a separate issue from the halter issue because Impressve who is the foundation of HYPP is also the beginnings of the modern halter horse look thanks in part TO HYPP. Folks are still breeding HYPP postive horses in some places on purpose hoping to get that appearance.Getting rid of HYPP will not change the judging about the other confo flaws that have now become that halter standard, but it is a step in the right direction for the health of the breed as a whole.