Spirit is Impressive bred all down the sires side of her pedigree but is N/N.
Impressive is notorious as the primary source of the widespread genetic disease known as Hyperkalemic periodic paralysis (HYPP). Although Impressive was not known to have exhibited any symptoms of the disease himself, gradually it became evident that many horses tracing to Impressive were afflicted with the painful, alarming, and often fatal disease Hyperkalemic periodic paralysis (HYPP). Further, it has never been observed in horses not descendants of his line. HYPP is a dominant gene, and as such, all animals with even one copy of the gene, identified as "N/H", will exhibits 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 that N/H horses.
After a number of years of debate, effective since January 1, 2007, the 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 to be homozygous for the condition (H/H).
However, other breed registries that accept animals with Quarter Horse bloodlines, including the American Paint Horse Association and the Appaloosa Horse Club, have yet to bar such animals. There is currently a widespread effort among many breeders to eliminate the disease by selective breeding, but there are those who continue to breed without regard for it, seeking the muscular enhancement correlated with it, and in doing so perpetuate the disease's existence.