Since I am on a roll with the genetics thing, there is a list of the 6 genetically linked...and lethal conditions that are prevalent today. If anyone knows of any others, please list them in the comments!! I'm not going to get too descriptive about them...cause there is info all over the net to read and digest.
HYPP-Hyperkalemic Periodic Paralysis...aka "The Impressive Gene". This disease orginated with one stallion-Impressive. This is the only genetic condition of the 6 that is not 100% lethal. There is a test. Horses that are N/N CANNOT pass the gene or the resulting condition down. N/H horses can pass the condition down and may or may not have symptoms. H/H horses are dominant for the gene and every single one of their foals will be N/H or H/H(I think that is right?). These horses are symptomatic and can die or will eventually die because of it. There is a test.
SCID-Severe Combined Immuno Deficiency. To date this condition only affects purebred Arabians, but there are studies that indicate that other breeds may also carry the SCID's gene. There is a test.
OLW-Overo Lethal White...aka "Lethal White Syndrome". Primarily this has been an affliction for the Paint Horse Association, but now that the AQHA has lifted it's maximum white rule, we are going to see problems with it in QH's now too. This is a recessive gene, so can only cause problems when 2 recessives are mated together. There is a test.
HERDA (or HC)-Hereditary Equine Regional Dermal Asthenia. This is linked to QH's that descend from Poco Bueno and I saw on one article...Doc O' Lena? This is a recessive gene. There is a test.
GBED-Glycogen-Branching Enzyme Disorder. This condition has been traced to Zantanon, through his son, King-P234. Only about 8% of the decendents of King carry the gene. I don't know how they can say that, because I suspect very few of the total number of descendants have been tested. This is a recessive gene. There is a test.
JEB-Junctional Epidermolysis Bullosa. This condition affects the foals of Belgins and American Cream Drafts. It's possible that it has been found in some saddlebreds as well. But it actually affects 25-30% of the population of these two breeds. I don't now if there is a test, but I presume there is.
Again, all of these diseases are 100% lethal, with the exception of HYPP.
JEB, GBED, SCID and OLW exclusively affect foals. All result in the eventual death of the foal.
HERDA may not be noticeable in all foals, but becomes apparent as the horse matures. I suspect that there are some horses who live longer than others, but inevitably they all eventually have to be euthanized, or should be(who wants to look at your horse with it's skin peeling off-gross!!).
The perpetuation of these known killer genetics ultimately comes down on the breeders. But I also feel that once a breed organization knows about them, registration of afflicted bloodlines(or color patterns) should be contingent upon the proper testing and all testing be stamped right on the papers. The breed organizations are afraid to force the issue for fear of driving members away. But if the AQHA, and every breed organization that recognizes the use of QH's for breeding purposes takes the same stance at the same time, people will either be forced to acknowledge their crappy breeding practices and stop or they will get out of the horse breeding business...a win, win in my book.
The one question that people may ask is why now? Why are all of these genetic disorders becoming so prevelant now? In my opinion, they have always been there. Almost all domestic animals carry recessive genes for some sort of genetic disorder. In cattle, they test for recessive genes by breeding bulls back on their own daughters. This normally brings any recessive genetic disorders to the surface and can determine whether a bull is kept or not. So right off the bat, they inbreed to check for genetic purity and then proceed forward. Linebreeding can hide the genetic disorder for generations. But eventually it will come out. Since the equine breeding programs have moved away from inbreeding...where recessive genetic disorders become immediately apparent to linebreeding, those recessive traits are only now becoming a problem. Another major problem, as I see it, it that there are a lot of breeders out there these days that just don't cull aggressively enough. Since the incorporation of the Foundation Quarter Horse associations...people have become more concerned with percentages than the actual physical attributes of the horses that they are putting into production. Mares that normally never would have been bred are popping out baby after baby and stallions that obviously should be gelded are siring tons of mediocre foals.
If you look at what the "foundation" breeders did in terms of inbreeding and linebreeding, it is pretty obvious that their intention was always to create a better horse than either the sire or dam. They were actively trying to perpetuate greatness. I have no doubt that they faced a lot of the same problems as breeders today, the difference is they were okay with sub-par individuals disappearing from the history books. Another interesting fact to note is that breeders used to recognize the importance of "the outcross". Inbreeding and linebreeding were okay, but when they got what they wanted, they sought out a complimentary outcross to create hybrid vigor. The resultant product often exceeded both the inbred parent and the outcross parent. That was the whole purpose.