On Jun. 9, 1973 Secretariat won the Belmont Stakes by 31 lengths and a time of 2:24 flat, which broke the stakes’ record by more than two seconds, another record that still stands.

Secretariat, the record setting winner of the Triple Crown in 1973, had a heart estimated to be 22 pounds or two-and-three-quarters times as large as that of the average horse which is approximately 8.5 pounds.

Secretariat’s large heart can be genetically traced to the legendary Eclipse, foaled on April 1, 1764 in England for the Duke of Cumberland. The larger heart is passed down the female line on the X chromosome.

Eclipse was a descendant of the Darley Arabian and his maternal grandfather was Regulus, who was by the Godolphin Arabian. He was an undefeated racehorse, with 18 starts and 18 wins.

Today, this genetic knowledge combined with ultrasound imaging and other technology helps breeders take advantage of the X Factor.