Circulatory System of The Horse - The "X Factor"

The "X Factor"

The X factor theory proposes that a single mutation within a gene located on the x-chromosome of horses that causes a larger than average heart. A larger-than 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 (8.2 kg/18 lb), and Secretariat (estimated at 10 kg / 22 lb). It is also thought 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 was developed over 40 years ago 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 theory was never scientifically peer-reviewed and studies on the test used indicate that the heart score has no correlation to future racing ability. In addition, the actual gene has not been identified, nor has its mode of inheritance been determined, the condition may even be influenced by multiple genetic factors.