Twins in cardiovascular genetic research.

Twin studies have been largely responsible for showing the effects of genetic variance on a quantitative trait. The model is based on the fact that monozygotic twins share all genes in common, whereas dizygotic twins are related as siblings and share "on average" half their genes. Environmental confounders are minimized because twin children are usually exposed to similar environments. Blood pressure was first shown to be heritable in a twin study. However, intermediary phenotypes, such as components of the renin-angiotensin system, sympathetic nervous system, renal function, and the facility in excreting electrolytes, are also heritable. The advent of molecular genetics has made twin studies more useful than ever because of the power of quantitative trait loci analyses. Recruitment of the parents of dizygotic twins greatly facilitates this effort. Gene loci linked to blood pressure, intermediate phenotypes, cardiac dimensions, lipid concentrations, and even components of the ECG have been identified. The use of single-nucleotide polymorphisms then allows for novel association approaches within the cohort. Twin studies will continue to gain in importance and utility, particularly in elucidating normal human genetic diversity.