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


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

General Information:

  • 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 minature 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.

Inheritance: autosomal-recessive

→ Only animals with two copies of the varaint (sa/sa) are affected. Animals with only one copy are clinically normal carriers (N/sa). 

Possible Genotypes:

Genotype: The horse is:
N/N normal. The horse has no copies of the genetic variant causative for SA and therefore cannot pass it on to its offspring.
N/sa a carrier. The horse is clinically healthy. The variant will be passed on to its offspring with a probability of 50%.
sa/sa affected. 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.

Test information: This test detects both deletions in the SHOX gene.

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.

Breeds affected:

Shetland Pony & Miniature Horse

Test #: H125

Samples: Blood, Hair

Special price: 125 € (net)

Dwarfism Panel (ACAN + SA) Combo-Package Special price: 180 € (net)

Time required:10-14 days

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

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