From the AQHA website: • Hyperkalemic periodic paralysis (HYPP) is an inherited disease that leads to uncontrolled muscle twitching or profound muscle weakness, and in severe cases, may lead to collapse and/or death.
• HYPP is listed as a genetic defect in AQHA's rules, along with Parrot Mouth and Cryptorchid conditions.
• To date, HYPP only has been traced to descendants of IMPRESSIVE, #0767246.
•HYPP can be treated through diet and medication in most cases.
• Possible results of HYPP testing are N/N, N/H and H/H.
HYPP is inherited as a dominant trait, which means a heterozygous (N/H) stallion or mare bred to a normal (N/N) horse will result in approximately half of the offspring being affected and half being normal. The rare homozygote (H/H) usually is severely affected with the disease and will pass the gene to its offspring 100 percent of the time.
An informative page: http://www.bringinglighttohypp.org/
This link has a good discussion of opinions on whether or not people should breed hypp horses. Included in the article is this quote: http://www.halterhorsecentral.com/hypp.htm
According to Dr. Sara Caldwell. DVM, Ocala Florida, a graduate of Florida State University, College of Veterinary Medicine, “The non riding myth is propagated by the veterinary teaching schools, We are actually taught and directed in school to discourage clients from riding HYPP horses in order to protect ourselves, veterinarians, from possible legal action, should an accident occur. Not at all for reasons of veterinary health or management, it is strictly a legal issue.”
That's a pretty sad statement in regards to veterinarians in general, wouldn't you agree?
My personal thought is that, just because you can't always see it, doesn't mean you should regard your hypp horse as breeding stock. Those who continue to pass on this gene, regardless of how mild it may be in their horse, are breeding just as irresponsibly as the backyard dog breeder who breeds an animal with poor hips. You may not see it on the outside, and you may win in the ring, but we certainly ought not be passing along poor genetics just to win.