bay roan, perlino, cremello, and white. In the past, spotted color patterns were excluded, but now with the advent of DNA testing to verify parentage, the registry accepts all colors as long as both parents are registered. Stock type A stock horse is a horse of a type that is well suited for working with livestock, particularly cattle. Reining and cutting horses are smaller in stature, with quick, agile movements and very powerful hindquarters. Western pleasure show horses are often slightly taller, with slower movements, smoother gaits, and a somewhat more level topline – though still featuring the powerful hindquarters characteristic of the Quarter Horse. Halter type Horses shown in-hand in Halter competition are larger yet, with a very heavily muscled appearance, while retaining small heads with wide jowls and refined muzzles. There is controversy amongst owners, breeder and veterinarians regarding the health effects of the extreme muscle mass that is currently fashionable in the specialized halter horse, which typically is and weighs in at over when fitted for halter competition. Not only are there concerns about the weight to frame ratio on the horse's skeletal system, but the massive build is also linked to hyperkalemic periodic paralysis (HYPP) in descendants of the stallion Impressive (see Genetic diseases below). Racing and hunter type Quarter Horse race horses are bred to sprint short distances ranging from 220 to 870 yards. Thus, they have long legs and are leaner than their stock type counterparts, but are still characterized by muscular hindquarters and powerful legs. Quarter Horses race primarily against other Quarter Horses, and their sprinting ability has earned them the nickname, "the world's fastest athlete." The show hunter type is slimmer, even more closely resembling a Thoroughbred, usually reflecting a higher percentage of appendix breeding. They are shown in hunter/jumper classes at both breed shows and in open USEF-rated horse show competition. Genetic diseases There are several genetic diseases of concern to Quarter Horse breeders: Hyperkalemic periodic paralysis (HYPP), which is caused by an autosomal dominant gene linked to the stallion Impressive. It is characterized by uncontrollable muscle twitching and substantial muscle weakness or paralysis among affected horses. Because it is a dominant gene, only one parent has to have the gene for it to be transmitted to offspring. There is a DNA test for HYPP, which is required by the AQHA. Since 2007, the AQHA bars registration of horses who possess