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An Impressive Legacy
 By mosquito   •   16th Jun 2010   •   9,515 views   •   8 comments
Sometimes a horse is truly special, and it makes a mark on its breed that will last for generations – names like Bold Ruler, Abercrombie, Prince Plaudit, Bask…every breed has one. But sometimes the mark these horses make comes with a curse.

An Impressive LegacyImpressive was an appendix Quarter Horse born in 1968, although he earned his way into full Quarter Horse registration by the age of three. By the age of six, he was a world champion, and was becoming well known for along running rivalry with another influential quarter horse stallion, Two Eyed Jack. Impressive was originally a racehorse, but pedal osteitis brought an end to his racing career, and he turned to halter.

In the breeding shed though, Impressive was far more popular than Two Eyed Jack, and went on to sire over 2,000 foals, including 30 world champions. At one point his owner turned down $300,000 for him – an astronomical sum for a horse in the 1970s. Great? Sounds like it. Here’s a handsome stallion with a lot of talent, and able to sire talent too. Surely it could only be good news for Quarter Horses that he made such a stamp on the breed?

Well, yes and no. But Impressive has left another legacy – a genetic disease called Hyperkalemic Periodic Paralysis (HYPP). What’s unusual, is that impressive is the only known source of HYPP – every case traces to him. Impressive himself never showed any sign of HYPP, although some people think that one of the reasons for his outstanding physique was the muscle overgrowth that can sometimes happen with HYPP. In any case, Impressive was the sole source of the genetic mutation that causes HYPP.

What is HYPP? If it occurs with symptoms, it is painful and often fatal. Basically, it causes an overstimulation of the muscle fibers by the nervous system. When symptoms hit, they can be mild or severe. Usually, horses show strange muscular twitching, which is often misdiagnosed. Some horses can survive for quite a while on medication, but most HYPP positive horses have shorter lives because of the constant medication and the difficulty in caring for them.

In severe cases, HYP can strike suddenly and without warning, causing near instant paralysis and even death. This is why HYPP positive horses are not safe to ride – many riders have been severely injured, and some even killed, from a horse having a HYPP onset falling, or collapsing and trapping them underneath.

No one knows why Impressive developed the HYPP mutation, but many believe it was because of intense inbreeding to the stallion Three Bars. Other horses descended from Three Bars do not carry HYPP, but Impressive has Three Bars appearing three times in his pedigree in the first four generations. Such heavy inbreeding always creates a risk of a damaging genetic mutation.
Genetic research has shown that horses with only one copy of the gene (known as ‘N/H’) will show some symptoms. Horses with full blown genetic HYPP, called ‘H/H’, will have more severe symptoms, may even die, and as HYPP is a dominant gene, they’ll always pass on the HYPP condition.

Given that Impressive had 2,250 foals, and nowadays it is estimated that there are over 100,000 descendants, HYPP is a fact of life for Quarter horse breeders and owners. It’s hard to find horses without at least one instance of Impressive, and it’s a big problem for Quarter horse breeders. HYPP is a dominant gene if a horse is H/H, all of their foals will be N/H or H/H and carry the gene. For an N/H horse, there is a 50% chance a foal will be a carrier.

What’s being done about it? It’s been a hot topic for the AQHA, given the success and popularity of Impressive. In 2007, the AQHA passed legislation requiring all descendants of Impressive to be tested for HYPP before they can be registered, and H/H horses are banned from the AQHA stud book. N/H horses can be registered, and many other breed registries will accept quarter horses without testing, such as the Paint stud books, although the Appaloosa and Palomino registries have recently begun to require HYPP-negative certificates.
Ultimately, the responsibility for preventing HYPP will rest with breeders, who need to use only stock that have been tested negative for HYPP.

Unfortunately, when the culprit is a horse as handsome and successful as Impressive, many breeders were reluctant to take the condition seriously and just kept breeding from HYPP positive animals. In fact, much of the spread of HYPP early on was the reluctance of breeders and registries to label Impressive as the source of HYPP.

Testing is the only way to be sure a horse has, or doesn’t have the HYPP gene. Theoretically, HYPP can cause excessive muscling, but this is not a reliable test, and can be replicated with training and exercise. Even more challenging is that HYPP symptoms might not show up until a horse is in its teens – by which time it may have been used for breeding and have several offspring. This is one of the reasons that Impressive has managed to have so many foals – many were into second or third generations before the facts about HYPP were known, and the HYPP test wasn’t even available until 1992.

Today, any horse descended from Impressive can – and should – be tested, and only N/N horses used for breeding. Hopefully with responsible ownership and breeding, one day no horse will have to suffer the pain and discomfort of HYPP, and no owner will have to suffer that disappointment and frustration. Only then will Impressive have a legacy for all the right reasons.

Impressive passed away in 1995, shortly before his 27th birthday.
An Impressive Legacy
An Impressive Legacy
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Excellent article!!!! Very well written and informative! I loved the video! Thank you for writing this!
  Jun 16, 2010  •  3,911 views
Magik Rose Farm  
appendix Quarter Horse born in 1968, although he earned his way into full Quarter Horse registration by the age of three (quote) Well, if that's not proof there is no such thing as a "purebred" QH, then I don't know what is. What percentage TB was he, exactly?
  Jun 16, 2010  •  3,926 views
Lil Booger  
its very interesting. i have a horse that is a deccendent from this horse. we were worried he had this gene but thank fully he doesnt! good artical!
  Jun 17, 2010  •  4,080 views
great article!
  Jun 17, 2010  •  3,894 views
Fantasy Farms  
Great article!! Most QH and a few paints are decendents of Impressive.
  Jun 17, 2010  •  3,875 views
S Q U I  
When my mom and I look at QHs pedigree, I always check if there is Impressive.
  Jun 21, 2010  •  3,962 views
Hypnotic Equus  
Thats really sad. But the part I hate is that a few years ago, when testing before registering wasn't required,people actually bred for the conditions sometimes extreme muscling. Thats why I'm not a fan of the industry.
  Jun 23, 2010  •  4,032 views
Rochelle Gates  
Pretty good article, but, the statement "Impressive himself never showed any sign of HYPP" is absolutely false. We all saw him fall to the ground in Tulsa at a show, many years ago. He had episodes at the same time of year, every year. It was a very well kept secret for a long time, until he was seen.
  48 days ago  •  3,883 views
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