"X factor" Circulatory system of the horse
eclipse, horse proposed source of x factor
the x factor theory proposes mutation within gene located on x chromosome of horses causes larger-than-average heart. larger-than average heart documented in high-performance thoroughbred, quarter horse, , standardbred racehorses. first seen in eclipse, @ 6.4 kg (14 pounds). large heart seen in phar lap (6.4 kg/14 lb), sham (8.2 kg (18 lb)), , secretariat (estimated @ 10 kg (22 lb)). proposed theory great producing mare pocahontas homozygous x factor. large hearts have been found in 4 major thoroughbred lines, descendants of eclipse: princequillo, war admiral, blue larkspur , mahmoud. many outstanding race horses such eclipse , secretariat noted being excellent broodmare producers failed produce male offspring ability of sires, theory gene carried on x chromosome meant stallions large hearts pass on trait via daughters.
the heart score, using electrocardiography, developed on 40 years ago describe correlation between qrs (intraventricular conduction time) complexes , performances of several elite versus average racehorses belief large heart correlated athletic ability. belief widespread , therefore high heart score can increase animal s worth in circles. however, x-factor theory never scientifically peer-reviewed , studies on ecg protocol used, indicate heart score has little correlation future racing ability. in addition, gene(s) associated cardiovascular dimensions , athletic performance have not been identified, nor has mode of inheritance been determined; condition may influenced multiple genetic factors.
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