Hereditary Diseases
On top of colour and markings, every horse carries seven health genes. These are named after real inherited horse conditions.
| Gene | Condition |
|---|---|
| HYPP | Hyperkalemic periodic paralysis |
| PSSM1 | Polysaccharide storage myopathy |
| HERDA | Hereditary equine regional dermal asthenia |
| GBED | Glycogen branching enzyme deficiency |
| HWSD | Hoof wall separation disease |
| CA | Cerebellar abiotrophy |
| LFS | Lavender foal syndrome |
Clear, carrier or affected#
Each health gene has two alleles. A is the faulty allele and n is the normal one.
| Genotype | Status | What happens |
|---|---|---|
| nn | Clear | The horse is healthy and cannot pass the condition on |
| An | Carrier | The horse looks healthy, but can pass the faulty allele to a foal |
| AA | Affected | A foal with this genotype is born with the illness |
Avoiding affected foals#
A foal is only affected if it receives the faulty allele from both parents. That means:
- Two clear horses (nn) can never produce an affected foal.
- A carrier bred to a clear horse will never produce an affected foal, but some foals will be carriers.
- Two carriers bred together have about a 1 in 4 chance of an affected foal.
The safest approach is to keep track of each horse's health genes before you breed, and never pair two carriers of the same condition.