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Skeletal Atavism (SA)


 → Abnormal bone growth of front and hind legs, resulting in short stature and leg deformities.

Instructions for ordering:

  1. Enter the information of the first horse for which you want to order the test
  2. Add to cart
  3. Go to cart to check out, or
  4. Enter the information for a second horse
  5. For sample collection follow the instructions here.
  6. You can use this template to send in hair samples.
  7. Please also send the completed order form together with the samples.

For a breeder or veterinarian discount please contact us before ordering.

Any information you include here is optional, but will be included on the certificate of results.

Enter the name of your animal
Enter your animal's date/year of birth
Enter the breed name
Enter the registration number if known
Enter the microchip number
Is there anything else you would like to tell us about your horse? Reason for testing, symptoms etc.?

General Information

Please be aware that by the conditions of the Skeletal Atavism (SA) patent license, the identity of the horse must be verified for CAG to perform the SA test. Samples may be collected by a veterinarian or studbook representative who can confirm the animal’s microchip identification.

  • The term “atavism” originates from the latin word “atavus” (“ancestor”) and refers generally to the reoccurance of ancient traits in recent species.
  • During the evolution of horses, the ulna and fibula fused, so that the modern horse has only small remnants of these bones.
  • Skeletal atavism results in development of both bones again and causes severe deformation of the normal bones and joints.
  • This disease leads to severe pain in the affected ponies and miniature horses and hence to euthanasia due to poor quality of life.
  • Two mutations in the SHOX gene cause SA. Ponies may have both mutations and/or variants in the ACAN gene causal for the ACAN dwarfism.

CAG holds an unlimited license to the Skeletal Atavism TestTM from Capilet Genetics.


  • Malformation of legs and joints
  • Short, splayed legs
  • Progressive lameness
  • Impaired movement
  • Progressive pain

Inheritance and Genotypes

→ SA is an autosomal recessive hereditary disease, meaning that horses with only one copy of the genetic variant (N/sa) are clinically normal carriers while horses with two copies of the genetic variant (sa/sa) are affected.

Genotype: The horse is: Effects:
N/N normal. The horse does not have any copies of the genetic variant causative for SA and therefore cannot pass it on to any offspring.
N/sa a carrier.
The horse is clinically normal. The genetic variant causative for SA will be passed on to its offspring with a probability of 50%. These foals will also be carriers (N/sa).
sa/sa affected. If the horse lives long enough to reproduce, the variant will be passed on to all offspring. All offspring will be carriers (N/sa).


  • Carriers may be bred to normal animals (N/sa x N/N) without any risk of producing affected offspring. The offspring should also be tested before breeding to determine if they are carriers or normal.
  • Breeding two carriers (N/sa x N/sa) is not recommended due to the possibility of 25% of the offspring being affected.
  • Affected animals (sa/sa) should not be used for breeding.


Rafati, N., Andersson, L.S., Mikko, S., Feng, C., Raudsepp, T., Pettersson, J., Janecka, J., Wattle, O., Ameur, A., Thyreen, G., Eberth, J., Huddleston, J., Malig, M., Bailey, E., Eichler, E.E., Dalin, G., Chowdary, B., Anderssson, L., Lindgren, G., Rubin, C.J.: Large Deletions at the SHOX Locus in the Pseudoautosomal Region Are Associated with Skeletal Atavism in Shetland Ponies. G3 (Bethesda) :, 2016. Pubmed reference: 27207956. DOI: 10.1534/g3.116.029645.  

Tyson, R., Graham, J.P., Colahan, P.T., Berry, C.R.: Skeletal atavism in a miniature horse. Vet Radiol Ultrasound 45:315-7, 2004. Pubmed reference: 15373256.  

Further information is available at Online Mendelian Inheritance in Animals.

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