Genetic factors in the electrocardiogram and heart rate of twins reared apart and together.

Important physiologic mechanisms have been thought not to exhibit large amounts of variability, due in part to the assumption that critical biologic functions will have evolved to an evolutionary optimum. The attainment of this optimum would necessarily eliminate individual differences in these variables. Using a sample of monozygotic and dizygotic twins reared apart since birth or early infancy, 12-lead electrocardiographic recordings and vectorcardiograms were obtained. Values of these variables for monozygotic and dizygotic twins reared together were obtained from other studies. Maximum likelihood tests of genetic and environmental components of variation for PR interval, QRS duration, QT interval and ventricular rate indicated a significant contribution of genetic effects (most heritabilities ranged from 30 to 60%), with a negligible contribution from common familial environmental effects.