There is a genetic disorder in some descendants of Impressive.

Not all Impressive descendants carry this genetic disorder.

Its called Hyperkalemic Periodic Paralysis.  HYPP for short.  It can cause strange muscle tremors, even leading to death (worse case scenario).  Some positive horses are symptomatic and some are not.  There is also a disorder of this kind in humans and pigs.

There is genetic testing for this disorder - you should be able to look on any AQHA papers and see if a horse carries this genetic disorder.  It will say:

 N/N - this horse does not carry the gene for this disorder (does not carry, cant pass it on, doesn't have it any more than that foundation horse you just looked at)

N/H - this horse carries the gene passed on from one parent, if you breed this horse to an N/N individual you have a 75% chance of having an N/N foal

H/H - this horse carries the gene passed from both parents, as of 07 this individual is no longer registerable.  You get this from breeding an N/H parent to an N/H parent.

If it doesn't say, and the horse is a descendant of Impressive and this is a concern for you, then ask the owner to have the horse tested before you buy.  When they get the test results back they come with a sticker that goes right on the registration papers.

There is a medication that horses positive for this gene and symptomatic can be put on that stops the symptoms.  I don't think this is true for HERDA - a genetic disorder carried in the Poco Bueno line of quarter horses...did that foundation breeder that just told you about the Impressive disorder educate you about HERDA?

As of May 9, 2007, Researchers working independently at Cornell University and at the University of California, Davis announced that a DNA test for HERDA has been developed. Over 1,500 horses were tested during the development phase of the test, which is now available to the general public through both institutions.[1]

The disease is found primarily in the American Quarter Horse, specifically in cutting horse lines. Affected horses have been found to trace to the stallion Poco Bueno, or possibly, farther back to one of his ancestors.[2] Researchers have now named four deceased Quarter Horse stallions that were carriers and produced at least one affacted HERDA foal; they are Dry Doc, Doc O'Lena, Great Pine, and Zippo Pine Bar. These stallions all trace to Poco Bueno through his son and daughter Poco Pine and Poco Lena. Other breeds affected are the American Paint Horse (APHA), and the Appaloosa (ApHC) and any other breed registry that allows outcrossing to AQHA horses.

HERDA is characterized by abnormal skin along the back that tears easily and heals into disfiguring scars. The skin is loose, and hyper-elastic in affected horses. Symptoms typically don’t appear until the horse is subjected to pressure or injury on their back, neck or hips, usually around two yrs of age. 

From Wikipedia, the free encyclopedia

Back to HYPP...

By all means research research research!  With the internet you can educate yourself!  My personal view is that I would like to breed this out...my halter horses mostly end up being riding and all around horses in the end (really PRETTY Riding and All around horses!!!)   so its just one more thing that folks would like to not have to deal with.  But I am also not going to throw the baby out with the bathwater, the few positive horses I have owned have also been the best I have owned and I carefully breed them to N/N.  You will find that if you have been looking at my horses and I have not said anything to you about HYPP, its because you have been looking at N/N individuals.

I believe in form to function, and the modern day halter horse to me is GORGEOUS and can do other things then stand around an look pretty, but I want em to do that too!  You will also notice on my website and ads that I state if the horse advertised is N/N or N/H.  Have fun horse shopping!