Some french trotters have difficulties in sustaining trot at high speed. See here why.

At Uppsala University, Sweden, Leif Andersson, Klas Kullander, Lisa S. Andersson and their research teams have found a high frequency of the gene mutation DMRT3 in horses bred for harness racing. These horses have the ability to trot or pace at high speed without breaking into a gallop, the natural gait at high speed for horses.

Klas Kullander

The American Standardbred was established in the 19th century and bred for harness racing. Competitions are held separately in trot or pace and assortative mating based on preferred gait has subdivided the breed into two populations, pacers and trotters.

In contrast to the pattern in Icelandic horses both Standardbred pacers and trotters are homozygous for the mutation. Thus, the mutation may promote the ability to trot or pace at high speed and genetic modifiers determine the gait to which the horse is best suited.

The Swedish Standardbred is largely developed from the American Standardbred but is not completely fixed for the DMRT3 mutation, probably owing to the import of French trotters, a breed with a fairly high frequency of the wild-type allele.

The segregation of the two alleles in the Swedish Standardbred provided an opportunity to examine the effect of the mutation on racing performance. The DMRT3 mutation was associated with superior breeding values (BV) for racing performance (BV_CA = 95.7 ± 1.7, n = 17; BV_AA = 109.0 ± 0.8, n = 206; P < 0.0001) and increased earned prize money (X_CA = 48,000 ± US$35,000, n = 17; X_AA = 161,000 ± US$24,000, n = 206; P_one-sided = 0.007).

The research team also genotyped 61 horses from one racing camp in a blind test; two of these had major difficulties in sustaining trot at high speed and were heterozygous C/A, whereas all others were homozygous A/A (P = 0.0005).

Leif Andersson

A description of the resarch project was published on the SLU website:
http://www.slu.se/en/about-slu/fristaende-sidor-eng/whats-on/news/2012/8/a-single-gene-has-a-major-impact-on-gaits-in-horses-and-in-mice/

The scientific report was published online 29 August 2012:
http://www.nature.com/nature/journal/v488/n7413/full/nature11399.html