MTSN - The Speed Gene
The variant for sprinting is written as "c" while the gene for stamina is written as "t".
This variant is also attributed to bulky muscles.
as an eg. Quarter horses were bred for speed, a lot of them are homozygous for the Sprinting Gene and their MTSN gene would read c/c.
To understand better we can look at how this knowledge is being used with Throughbreds, who also have a high portion of c/c horses. Bred specifically for racing as well, we can look at the following three individuals:
The c/c horse would be best suited to short races.
The c/t horse would be best suited for longer races.
And the t/t horse would be best suited for long endurance races.
Another example would be Arabians - known for endurance. Purebred Arabians are t/t and have no sprint genetics.
One thing that is certainly agreed on is tha tthe above is not a cut and dried answer for breeding speed or stamina into horses. There are more speed genes and gene combinations for speed to be discovered.