THE INHERITANCE OF HEART SCORE IN RACEHORSES

Abstract

SUMMARY Heart scores derived from the ECG's recorded from sires, dams and progeny have been used to determine the heritability of heart size in Thoroughbred and Standard‐bred horses. Regression of progeny on male parents gave an average heritability of about 0.4. The heart scores of progeny reflected the heart scores of dams more closely than those of sires. Although this result raises questions which cannot be answered completely from the data available, the heritability estimate of 0.4 should enable breeders to improve heart size by simple selection of breeding stock on heart score. The greater influence of dams on heart score could involve sex differences, maternal environmental factors and possibly sex‐linkage. In addition, in the population studied there was unequal selection of sires and dams. Critical selection of mares seems to offer the largest potential for improvement in the future, but this will be inhibited by practical restraints on the culling of the brood mare population.