The sheer number of horses being sold with confirmed HYPP is crazy to me, they practically advertise it with a lot of these backyard bred horses. You know it's a fucked up business when sellers aren't even attempting to hide deformities and serious health conditions in the animals they're breeding. I guess people aren't being outright scammed by omission, but it's still bizarre. It would be like these "Bully breeders" advertising confirmed entropion or tracheal collapse as a "feature" and selling them full price.
It makes them beefy and muscular because the muscle fibers are constantly twitching. Easier and quicker than pumping them with steroids or actually, you know, exercising them (risky because halter conformation means muscle building exercise is really rough on their toothpick legs and teacup sized hooves).
HYPP is an incomplete dominant. One dose of the HYPP gene is less severe than two doses. One dose or two, ALL animals with the gene are affected. Owners of a heterozygous HYPP horse will claim that it's "unaffected" or "easily managed" through medications and a lifelong special diet. Put your kids on a horse like that and you're risking their lives.
It's not like a recessive gene like OLWS (lethal white) or HERDA, where an animal with one copy of the gene is perfectly healthy, but one with two copies either dies painfully shortly after birth (OLWS) or walks right out of its skin (HERDA). Just don't breed carrier to carrier, and there's no risk of a lethal white foal. Backyard horses (of course) also often have the gene, and this results in more lethal white foals than there needs to be.
Genetic traits are fun. Cream (palomino / buckskin) is another incomplete dominant. Any red or bay horse with one copy will show the trait visibly. Two copies produce a nearly white horse with pale skin and eyes. For decades, the AQHA refused to register these horses because they had too much white. They're also IMO unattractive and susceptible to sunburn and skin cancer unlike their black-skinned colleagues.
The "Silver Dapple" gene is another incomplete dominant color gene with bad effects, but people like the weird colored horses it produces. It fucks up the structure of their eyes as well, one copy of the gene not as severely affected as two copies. But they sure are purty. There is (or was) a Quarter Horse stud named "Silver Assured" who was homozygous and therefore reliably passed the gene to all his babies. There's a decent chance he's blind.
Appaloosas (the ones with anything from a spotty butt to overall leopard spots) also have fucked up eyes. This one is polygenic with multiple alleles, and the right combination causes a painful disease where the eye periodically gets inflamed and swollen eventually causing blindness. But spots sure are pretty so.
Just about every pet species has genes that can really mess up an animal. Some white spotting genes are almost always associated with deafness. In fact, genes that add white to the coat are disproportionally likely to cause developmental problems, which is a bummer because they're also really cute. Some are perfectly healthy, a lot of them are not.
Certain coat color genes (merle in dogs -- think Aussie Shepherds, various "roan" and "mottled" genes in guinea pigs and hamsters, and probably lots of other animals) cause a pretty coat pattern with only one copy of the gene, and blind (or even eyeless), deaf, and often toothless babies with a double dose.
NATURALLY, dog breeders are falling over themselves to introduce the merle gene into pitbulls, french bulldogs, poodles, and other breeds where it has no business existing. This has the Kennel Club people a-tizzy, because Blood Must Be Kept Pure At All Costs which is why ALL Dalmations can't get rid of uric acid. Meanwhile breeders are studding out double-merle dogs that are eyeless and deaf, and almost pure white, because OF COURSE THEY ARE, so you'll always get merle pups.
The squashed brachycephalic faces are far from only the obvious genetic crime against dogs (and cats). There are just
so many other genes that look cool on the surface but cause one degree or another of suffering to the animal.
Hairless mutations are cute (YMMV), but females often can't nurse their young because the same defect also causes dysfunctional mammary glands.
Curly haired mutations are cute, but for those who absolutely NEED their whiskers to get around (nocturnal, tunnel dwellers, etc.), this deprives them of a critical sense organ.
Long haired mutations are cute, but diabolically incapable of keeping their luxurious hair properly groomed. They get all matted up and suffer from literal skin rot and bug infestations if not cared for rigorously by owners. Bonus: long hair and flat faces is the ultimate torture for Persian cats and related breeds when the weather gets hot.
Dwarfism is cute (again YYMV) but long-backed dog breeds like Doxies have well documented back problems.
Short or missing tails are cute but too short and they have no bowel or bladder control (oops) and sometimes their back legs don't work right. Healthy ones born with tails get them docked anyway so they look right.
There's a special place in hell for people who like cats with folded ears. The floppy ears are from a deformity in the cartilage which also (guess what) affects the cartilage in all their joints, leading to crippling arthritis. Worse, of course in homozygous victims, but heterozygous ones are still universally affected, just not as crippled.
There is zero reason to breed animals with traits that adversely affect their health. There is zero reason to eliminate "wrong colors" or "mismarks" from the breeding pool (Friesian horses) when the pool is already so inbred that dwarfism and dystocia are just business as usual.
None of these traits give animals any legitimate advantage in their "jobs". None of them give the tiniest fuck how pretty they are. A large proportion of them would be incapable of surviving without human intervention.
But keep on breeding those smashed n slammed toadline microbullies, folks, and flogging frozen dog jizz so the sire who died at 1.5 years old can continue to make his mark on this exciting new breed.
Did I mention I fucking hate animal breeders?
