The X factor theory proposes that a mutation within a gene located on the X chromosome of horses 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 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.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.