Different breeds of horses are suited for different tasks. Some of the differences between breeds are explained by genetic variants that are well understood, while other differences between breeds result from genetic differences that have not been well studied. One example of a simple genetic difference between breeds is the different racing abilities of Quarter Horses and Thoroughbreds. Quarter Horses are bred to run the quarter mile, and are like human athletes that are sprinters. They get up to speed very quickly but don't perform well over long distances. Thoroughbreds, on the other hand, are bred to run longer distances, like the Kentucky Derby at 1¼ miles.

Most of the difference between "sprinters" and "stayers" is the result of variant forms of a single gene. Genes are units of heredity that are made of DNA. The function of a gene is to direct the synthesis of a protein. Slight differences in the exact makeup of a particular gene between individuals (differences in the DNA sequence of that gene) result in differences in observable traits. The gene that determines the difference between sprinters and stayers encodes a protein called myostatin. The function of myostatin is to regulate the amount of muscle development. The version of the myostatin gene found in Quarter Horses allows the development of more muscle mass than the version found in Thoroughbreds. The larger muscle mass of a Quarter Horse allows it to burst out of the starting gate and reach top speed quickly. However, it is difficult to haul that muscle mass over a mile, so a Thoroughbred will usually beat a Quarter Horse when the race is over a mile.