average heart was documented in certain high-performance Thoroughbred, Quarter Horse, and Standardbred racehorses. It was first seen in Eclipse, at 6.4 kg (14 pounds). A large heart was also seen in Phar Lap (6.4 kg/14 lb), Sham (), and Secretariat (estimated at ). It is also proposed as a theory that the great producing mare Pocahontas was homozygous for the X factor. Large hearts have been found in four major Thoroughbred lines, all descendants of Eclipse: Princequillo, War Admiral, Blue Larkspur and Mahmoud. Many outstanding race horses such as Eclipse and Secretariat were noted for being excellent broodmare producers but generally failed to produce male offspring with the ability of their sires, thus the theory that the gene was carried only on the x chromosome meant that stallions with large hearts could only pass on the trait via their daughters. The Heart Score, using electrocardiography, was developed over 40 years ago to describe the correlation between the QRS (intraventricular conduction time) complexes and the performances of several elite versus average racehorses with the belief that a large heart correlated to athletic ability. This belief is widespread and therefore a high heart score can increase the animal's worth in some circles. However, the X-Factor theory was never scientifically peer-reviewed and studies on the ECG protocol used, indicate that the Heart Score has little correlation to future racing ability. In addition, the gene(s) associated with cardiovascular dimensions and athletic performance have not been identified, nor has its mode of inheritance been determined; the condition may be influenced by multiple genetic factors. Disorders of the circulatory system Anemia Arrhythmia Heart disease Heart failure References External links Equine Anatomy and Physiology: Circulatory Systems Horse anatomy