Impressive (horse)
Hyperkalemic periodic paralysis
Hyperkalemic periodic paralysis
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HYPP was first recognized in the 1980s, but it became evident that many horses descended from Impressive were afflicted with the genetic disease [hyperkalemic periodic paralysis](/wiki/Hyperkalemic_periodic_paralysis_%28equine%29 "Hyperkalemic periodic paralysis (equine)") (HYPP), and in 1992 Impressive was publicly linked to HYPP and considered the [index case](/wiki/Index_case "Index case"). As of 2009 there were 355,000 registered descendants of Impressive with approximately 4% affected with HYPP. 

HYPP is a [dominant gene](/wiki/Dominant_gene "Dominant gene"), and as such, all animals with even one copy of the gene, identified as "N/H", will exhibit some symptoms of the disease. Horses with two copies, identified as "H/H." will always pass on the condition, and research suggests that H/H horses may have more severe symptoms than N/H horses.

After a number of years of debate, effective since January 1, 2007, the [AQHA](/wiki/AQHA "AQHA") amended rule 205(c)(3\) and rule Rule 227(e) to require all descendants of Impressive to be tested prior to being registered, and ban from registration all horses born after January 1, 2007 with HYPP genetics confirmed by [DNA testing](/wiki/DNA_test "DNA test") to be [homozygous](/wiki/Homozygous "Homozygous") for the condition (H/H). Other breed registries that accept animals with Quarter Horse bloodlines, including the [American Paint Horse Association](/wiki/American_Paint_Horse_Association "American Paint Horse Association") and the [Appaloosa Horse Club](/wiki/Appaloosa_Horse_Club "Appaloosa Horse Club"), have instituted testing requirements but have yet to bar animals with the condition. There is currently a widespread effort among many breeders to eliminate the disease by [selective breeding](/wiki/Selective_breeding "Selective breeding"), but there are those who continue to breed without regard for it, or even deliberately seeking the muscular enhancement correlated with it, and in doing so perpetuate the disease's existence.


What is the effect of a horse being homozygous for HYPP?