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, aother 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 in England for the Duke of Cumberland in 1764. The larger heart is passed down the female line on the X chromosome.

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