Gene or region: ACAN, B4GALT7
Reference allele: various
Mutant allele: various
Affected Breeds: Many
Research Confidence: Cautious optimism; findings require multiple studies and further validation.
What it does:
Dwarfism is characterized by abnormal growth, leading to either a disproportionate body type or overall smaller body size. There are many variants for Dwarfism including, but not limited to, ACAN 1-5 (originally found in miniature horses, and Shetland ponies), and B4GALT7 (originally founded in Friesian horses). Dwarfism caused by mutations in the Aggrecan gene (ACAN) results in a dwarf foal with a wide range of physical ailments and limitations. Dwarf horses often have breathing problems, eating difficulties, and other health issues related to their abnormal bone growth. Other unrelated causes of Dwarfism are skeletal atavism, and osteochondrodysplasia which are not a result of the ACAN mutations. Specifically in the Friesian, the limbs and ribs are mainly affected, leading to a normal sized head, disproportionate back, and abnormally short legs. Research has indicated that it is inherited in an autosomal recessive manner. A recent study also indicated that horses with one variant may be 20% smaller than their ACAN negative relatives.
A single genome-wide association study of 10 dwarfs and 10 full-sized Friesians failed to identify a candidate gene or mutation. The tested SNP had the strongest association within this study, though the researchers did not report which allele was linked to the dwarfism. Further research is warranted to test this association.