Genetic Basis of Gait Variation Uncovered in HorsesBy Kentucky Equine Research Staff · December 14, 2012
The walk, trot (or jog), and canter (or lope) are referred to as the three natural gaits of horses. Many horses have a preferred mid-speed gait that is different from the diagonally patterned trot. This may be a laterally patterned pace, a four-beat gait such as the amble, or one of a number of variations on these gaits.
Recent research has identified a premature codon stop in the DMRT3 gene that has a major effect on gait styles. This mutation allows some horses to show alternate movement patterns that may be enhanced by training methods, and gadgets. Horses without the mutation are unable to perform these gaits, regardless of training.
The pattern of locomotion is established by a circuitry of spinal interneurons developed in the fetus before birth. This nerve network produces left-right alternation of limbs and coordinates flexion and relaxation of muscles used in walking and the faster gaits. The mutation changes these stride-controlling nerves, allowing different sequences of limb movement.
Horses that have one or more alternate gaits are known collectively as gaited horses. Their patterns of leg movement are not all the same, and minor variations of movement have led to the development of different horse breeds known for their unique gaits. The Icelandic Horse performs the tolt, a single-foot gait that is fast and smooth. Some Icelandic Horses also pace in addition to being able to walk, trot, tolt, and canter. When 352 five-gaited Icelandic horses were genotyped, all but one were found to be homozygous (received the gaited mutation from both parents) for the change in the affected gene. Horses can also be heterozygous (receiving the mutation from only one parent), in which case they may or may not be able to produce alternate gaits.
Standardbred horses in America are bred for harness racing, a sport in which horses pull a lightweight sulky at high speeds. Some Standardbreds prefer to trot, while others have a tendency to pace. Testing showed that a high percentage of Standardbreds are homozygous for the DMRT3 mutation, which allows but does not mandate the ability to pace.
Swedish Standardbreds were developed mostly from American Standardbreds, but the breed has also been influenced by crossing with horses that have French Trotter pedigrees, most of which do not have the gait mutation. In one blind test, 61 Swedish Standardbreds were asked to perform a trot at high speed. Two of the horses had difficulty in sustaining the fast trot. These proved to be heterozygous, while the horses that trotted more easily at high speeds were all homozygous. This illustrates the fact that the mutation not only allows various alternate gaits, but also influences coordination of the gaits, especially at faster speeds.
The discovery of the DMRT3 gene does not completely explain why some horses only trot while others are able to perform varied gaits. In some horses, the mutation may be incompletely expressed because of other genetic determinants that are not yet known. Gaitedness, the exact pattern of limb movements, and the skill and speed with which a gaited horse moves may also be strongly influenced by training, maturity, and other factors.