SCN4A (HYPP)

Crossing two of them

Its gene carrot

There is none. A disorder locus sets hasGeneCarrot() false — forcing one onto somebody’s line is hostile — and SpliceSafety keeps it out of the random splice as well, derived from the fact that it declares a non-informational Condition rather than from any hand-written list.

Gene key

horsegenetics.scn4a

Priority

93

Alleles

H N

Outcomes

wild, hypp, hypp-lethal

Coat effect

none — every outcome is a wild type

Default allele

N

Wild population

1.2% of founders are born H/N — and therefore born affected

Founder draws

1 × nextFloat()

Catalogue entries

1 — it never changes what a horse looks like

The combination table

The first locus that is both

Every disorder before this one was a single thing: a heart reduction, or a lethal. HYPP is a sub-lethal heart reducer and a homozygous lethal on the same locus, which is a shape nothing in the mod had — and the reason DominantDisorderGene exists alongside RecessiveDisorderGene.

The consequence for a breeder is the point of the gene. An H/N horse is visibly unwell and can still be excellent on every other axis, so it is genuinely tempting to breed from — and two of them throw a dead foal one time in four while half the survivors are affected again. It is the only locus in the mod where the mistake is one you make with your eyes open.

The numbers are a rendering, not arithmetic

The reference table gives HYPP as roughly “30% health, 60% speed and jump”, in a severity model this mod does not have — roadmap §3 flags it and does not decide it. Read literally against a 22-health baseline, those percentages would make an affected but living horse worse than several of the mod’s outright lethals, which is plainly not what the table means.

So the figures here are calibrated against the disorders that already ship — the worst survivable outcome in the mod, and comfortably short of a lethal — and the reference is used for the ordering of the disorders against each other, not for the arithmetic. Every other new health locus follows the same rule.

Core facts

What the channel does, and what the mutation stops it doing

Skeletal muscle contracts by creating and propagating an electrical impulse. Nav1.4 is central to it: a channel opens briefly and lets sodium into the cell; the inward current depolarises the membrane and starts an action potential; the channel rapidly inactivates, shutting sodium entry off; potassium movement then returns the membrane to rest, ready for the next controlled contraction.

F1416L impairs that inactivation step. Channels stay open too long or reopen inappropriately, giving excess sodium entry and a persistently depolarised, electrically unstable membrane. UC Davis VGL

The instability has two phases, and they look like opposite diseases:

• Early, excitable: fasciculations, twitching, stiffness, myotonia, sometimes spasm.
• Later, inexcitable: if sustained depolarisation goes far enough, the channels cannot reset at all. Fibres stop firing — weakness, wobbliness, inability to rise, collapse, flaccid paralysis.

Serum potassium is often elevated during an episode. Hyperkalaemia both worsens the membrane instability and makes repolarisation harder, and spontaneous muscle activity moves more potassium out of cells — so the attack reinforces itself. Vetlexicon

The clinical picture

Episodes last minutes to hours. Horses can look completely normal between them, and some genetically positive animals have few or no observable events under ordinary management — which does not make them clear, and does not stop them transmitting.

Signs include fine muscle twitching over flank, shoulder, neck or face; prolapse of the third eyelid; stiffness or a locked-up appearance; weakness, stumbling, hindquarter weakness or abnormal gait; sweating, an anxious look, respiratory noise; recumbency, inability to rise, partial or generalised paralysis; and rarely sudden death from severe respiratory-muscle involvement or an arrhythmia with marked hyperkalaemia.

HYPP is often first noticed in young horses, but the age of recognition varies, and so does severity — the same genotype looks different across individuals and within one horse depending on diet, stress and management. A horse showing tremors, weakness or collapse should not be assumed to have HYPP on appearance: colic, exertional disorders, neurologic disease, toxicities and other electrolyte abnormalities all overlap. Potassium and acid-base testing help during an acute episode; the DNA test settles whether the allele is there.

Inheritance

Autosomal, so sex is irrelevant, and each conception is independent. Dominant, so N/H is not a benign carrier state — there is no silent carrier at this locus at all, which is the structural difference between HYPP and every recessive on the health layer.

The disease lineage traces to the influential AQHA stallion Impressive (born 1969); every horse identified with the allele in an early study descended from him. That HYPP spread despite its cost is a standard population-genetics story: a heavily used sire disseminates an allele fast, especially when breeders associate his line with a desired show phenotype. AQHA requires testing for horses descending from Impressive or other designated positive bloodlines. AQHA

The homozygote, and what the mod does with it

H/H horses exist, live, and are generally at higher risk with earlier onset and more severe disease. They pass the allele to every foal. Since 2007 they cannot be registered with AQHA, which is a rule about paperwork rather than about survival.

Coat, build, size, performance

Coat. No evidence that the allele determines or predicts black, bay, chestnut, grey, dun, cream, champagne, roan, pearl or silver; tobiano, overo, sabino, splash, leopard complex or any white pattern; or eye, skin, mane and tail colour. It is a muscle ion channel, not a pigment locus.

The halter-horse claim. Some breeders favoured an unusually heavily muscled look in Impressive descendants, which produced a persistent belief that HYPP itself causes exceptional muscling. The evidence does not establish F1416L as a muscling gene. Affected horses show intermittent stiffness, prolonged contraction and abnormal muscle activity — that is pathology, not hypertrophy or desirable athletic muscle physiology. The historical concentration in halter stock is far better explained by sire-line use and selection around one family. OMIA notes that breeders selected the associated muscular phenotype; that is not proof the mutation causes it. OMIA 000785

There is likewise no reliable evidence that the allele determines mature height, weight, bone, body size, head shape, neck set, hip angle, limb proportion, hoof structure, baseline muscle-fibre type, soundness or a reliably desirable build.

Speed. No robust evidence supports using HYPP genotype to predict sprint speed, racing success, stride length, acceleration, earnings or endurance. One could speculate that altered excitability changes contraction behaviour — but uncontrolled excitability is exactly what makes HYPP dangerous, and during an episode weakness, stiffness, impaired coordination and paralysis plainly reduce performance and endanger horse and rider. A symptom-free N/H horse has no genetic speed advantage from HYPP.

Jumping. No validated association with scope, carefulness, technique, bascule, take-off power, landing or competitive results. The real point is risk management: HYPP is potentially incompatible with any situation where sudden weakness or paralysis endangers horse and rider — jumping, eventing, riding out alone, racing, transport. That is individualised veterinary judgement, not a blanket bar. The mod’s −0.040 speed and −0.18 jump on the heterozygote are the mod’s way of saying “this horse is not reliable” in the only vocabulary the trait layer has.

Breeds and frequency

The mod puts WILD_AFFECTED_PERCENT at 1.2 — the lowest number on the health layer, and deliberately so, because at this locus every positive founder is sick rather than merely carrying something. Founders have to be able to be affected here: a dominant with no silent carrier that never appeared in a founder could never appear at all.

Management

The aim is to avoid abrupt rises in blood potassium and the stressors that precipitate episodes. Reported triggers include high-potassium feeding or sudden dietary change, fasting, illness, transport, cold, anaesthesia and restriction of regular exercise — and they differ between individuals. Usual principles: a consistently low-potassium ration designed with a vet or equine nutritionist; avoiding sudden large alfalfa meals where relevant; regular turnout or exercise rather than long confinement; routine, minimal stress, no fasting; telling the veterinary team the HYPP status before sedation, anaesthesia, hospitalisation or emergency treatment; and an individualised emergency plan for a horse that has ever collapsed or shown marked weakness. Acetazolamide is used in some horses, as a veterinarian’s decision rather than a substitute for testing, diet and management.