Aerobic- this is work that can be sustained by the respiratory system that fuels muscles by an oxidative process. Less strenuous work that can be carried on for long periods of time with a low to medium heart rate is aerobic in nature.

Allele- these are different genes that affect the same characteristic in an unlike manner. They are usually characterized as dominant or recessive when there is a complete dominance but this can vary when other modes of genetic action influence that particular gene.

Amblers/Pacers- pacers are horses that pace instead of trot at the medium speeds. This means pacers will have a two-beat gait with lateral symmetry, in which the foreleg and hind leg of the same side will have identical suspension and weight-bearing phases in their stride.  Ambling horses are smooth-gaited horses that have a gait that is very similar to the pace, except there is a fraction of difference between the suspension and weight-bearing phase of the foreleg and hind leg of the same side, which gives rise to a four-beat gait with very short intervals between beats. 

Anaerobic- this is work that is done without oxygen that is fueled by enzymes in which lactic acid is produced as a byproduct. Work that is very demanding in speed or strength that cannot be maintained by accelerated breathing alone is anaerobic in nature. By definition, this type of work can only be sustained for short periods of time with elevated heart rates.

Anagenic transformation- this is when a genus that is progressively transformed into another one that is better adapted and disappears shortly thereafter.

Anestrous- this is the time of year when there is a permanent lack of sexual receptiveness to the male due to the fact that the reproductive functions of the mare are shut down in response to a shortened photoperiod (hours of daylight) during the winter months.

Anhydrosis- this is the inability to sweat. Approximately 25 percent of the horses raised in the hot humid tropics suffer from total or partial anhydrosis, which radically limits their ability to perform.