HYPP
Hyperkalemic Periodic Paralysis
dominant traitWhat is HYPP?
HYPP (Hyperkalemic Periodic Paralysis) is a muscle disease caused by a specific genetic mutation that originated in the Quarter Horse sire Impressive. The mutation disrupts sodium channels in muscle cells, causing episodes of weakness, sweating, muscle trembling, and in severe cases, respiratory distress or collapse. Because it is a dominant condition, a horse needs only one copy of the mutation to be affected.
How is it inherited?
HYPP is autosomal dominant — a single copy of the H allele is enough to cause clinical signs. Horses with two copies (H/H) tend to experience more frequent and severe episodes than horses with one copy (N/H). AQHA no longer allows H/H horses to be registered.
What do the results mean?
| Result | Status | What it means |
|---|---|---|
| N/N | Normal | Two normal alleles. Horse does not carry the HYPP mutation and cannot pass it to offspring. No risk of episodes. |
| N/H | Heterozygous (affected) | One normal allele and one HYPP allele. Horse is affected and may experience episodes. Will pass the mutation to roughly half its offspring. |
| H/H | Homozygous (severely affected) | Two HYPP alleles. Episodes are typically more severe and frequent. AQHA does not permit registration of H/H horses. |
Two normal alleles. Horse does not carry the HYPP mutation and cannot pass it to offspring. No risk of episodes.
One normal allele and one HYPP allele. Horse is affected and may experience episodes. Will pass the mutation to roughly half its offspring.
Two HYPP alleles. Episodes are typically more severe and frequent. AQHA does not permit registration of H/H horses.
What should buyers know?
N/N horses are clear and safe. N/H horses can be managed with a low-potassium diet and consistent monitoring, but require an informed commitment. H/H horses carry the highest health burden and registration restrictions — most buyers should proceed with caution.
Which breeds are affected?
Primarily seen in American Quarter Horses, Paints, and Appaloosas with Impressive bloodlines. Prevalence is highest in halter-type conformation lines.
This page is for general education only. Always consult a veterinarian or equine geneticist before making purchasing or breeding decisions based on genetic test results.
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