Testing Available for Dwarfism Gene in Miniature Horses

Testing Available for Dwarfism Gene in Miniature Horses

An example of normal Miniature Horse conformation.

Photo: Thinkstock

The University of Kentucky Animal Genetic Testing and Research Laboratory (AGTRL) is now offering DNA-based tests for four mutations in the aggrecan gene (ACAN) associated with dwarfism in Miniature Horses. John Eberth, MS, a PhD student of Ernie Bailey, PhD, professor in genetics and genomics at the UK Gluck Equine Research Center, discovered the mutations.

It is important to note that these mutations are not associated with another type of dwarfism known as skeletal atavism seen in Miniature Horses and Shetland Ponies. The ACAN mutations are also not associated with the osteochondrodysplasia dwarfism found in some breeds such as Friesians.

A horse that is a carrier for any one of the four ACAN mutations appears to be normal and does not exhibit any dwarf traits. Because carriers have the normal phenotype (physical characteristics), it is important to test breeding stock for these mutations to avoid matings that might produce a dwarf or aborted/absorbed fetus. One of these mutations (D1) is lethal in combination with any of the other mutations and will cause early pregnancy loss. Breeders should also avoid mating two horses that are carriers for the other ACAN mutations, as two mutations in any combination will produce a dwarf foal with a range of physical ailments. Some of these defects seriously affect the horse's health and include breathing problems, malformed mouths that cause eating difficulties, and abnormal bone growth leading to chronic soundness issues.

The four identified mutations are 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:

Normal phenotype Dwarf phenotype Lethal (aborted/absorbed fetus)
N/D1 D2/D2 D1/D1
N/D2 D2/D3 D1/D2
N/D3 D2/D4 D1/D3
N/D4 D3/D4 D1/D4

From top, a D2/D2 dwarf, a D2/D4, and a D3/D4 dwarf.

Photo: Courtesy Dr. Kathryn Graves

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

Again, because of the serious health problems associated with the dwarf phenotype and the potential for aborted pregnancies, researchers strongly recommend against breeding carriers to carriers.

Carriers can be used in a breeding program safely if they are only bred to horses with normal genotypes (N/N).

Information on test price and instructions for sample submission is available at www2.ca.uky.edu/gluck/AGTRL.asp#Dwarfism. For questions regarding the dwarfism test, please contact Kathryn Graves, PhD, assistant clinical professor and director of the AGTRL, at ktgraves@uky.edu or 859/218-1193.

Kathryn Graves, PhD, assistant clinical professor and director of the University of Kentucky A Genetic Testing and Research Laboratory, provided this information.

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