Certain wrote:So if the X-Factor is not factual then why in th last 15 years Keenalnd started making all horses not only get X-Rays, Scoped, and now have a Heartscore to enter the sale ring? What helped Marianna therory was when they tested ECG most of the daughters had there sire's heartscore. When they look at the sons tested most did not. The only thing they had to go on was pedigree of the TB. They have tested thousands of TB with ECG's now still today some have large hearts and some do not. Breeders want to breed a winner to make the stallion look good. Some people believe that the heartscore (large heart) makes up 25% of the race horse. For those that do how do you find the large heart to breed for it. X-Factor is one way that helps find the large heart size.
It's Haun's hypothesis about how heart size is transmitted that's disproven. No one's claiming that cardiovascular capacity is anything other than highly useful information for making buying and breeding decisions. However, the major providers of CV evaluations for TBs entering the auction ring are using different methodology than Haun's to qualify and quantify more specific and useful benchmarks when generating heart scores. They've reported no findings that indicate heart size inheritance is under any control other than autosomal (non x-linked).