ACAN Dwarfism Mutations in the Miniature Horse

Four mutations in the ACAN gene have been found in Miniature Horses associated with a dwarf phenotype by John Eberth, MS, working in the lab of Dr. Ernie Bailey at the Gluck Center. It is important to test breeding stock for these mutations because at least one of these mutations is lethal in combination with any of the other mutations causing early pregnancy loss. Care must also be taken in breeding two horses together that are carriers for Dwarf mutations, as the presence of two mutations in any combination will lead to early abortion or live foals with a range of physical ailments associated with the dwarf phenotype. Some of these physical ailments seriously affect the health of the horse and include breathing problems, malformed mouths which lead to eating difficulties, and abnormal bone growth leading to chronic soundness issues.

Horses carrying only one of the mutations are normal in appearance, exhibiting the desired stature and proportions the Miniature Horse should possess.  There are four identified mutations, designated D1, D2, D3 and D4. The normal copy of the gene is designated as N.

The following table summarizes the effects of the various mutation combinations:

Examples of Normal Phentypes

Examples of Dwarf Phenotypes

The genotypes D3/D3 and D4/D4 have unknown effects, as no samples with these genotypes have been found so far.

D1 and D2 were the most common mutations found in the pilot study.  Since D1 is lethal in combination with any of the other mutations, it is recommended that D1 carriers (N/D1) only be bred to horses that do not carry any other Dwarf mutation.

Testing: Hair samples may be used for the DNA test for these mutations.  Please see Hair Sample collection instructions on the submission form.  Make sure there are plenty of hairs with bulbs submitted if more than one test is requested. Fees: Full panel of all four mutation tests: $150; any test individually: $40 each.