big heart horses - good performers

Question
hello ma'am. i heard that horses that have bigger hearts are good performers. is it true? may i know how to pick a horse has a bigger heart?

Answer
Roberts,
  There is a thing called an X factor that leads to over sized, healthy hearts. There are several books that can explain it much better than I can. Since it is located on the X chromosome, it must be passed on through the dam, and a sire can only pass it to his daughters.
  When most people talk about horses with large hearts or a lot of heart, it usually has nothing to do with the actual heart. It has to do with how much courage a horse has. Unfortunately, there is no way to know this till your horse faces some adverse conditions.
Anne Stepien