INTRODUCTIONVariance component methods have a long history in human quantitative genetics as well as in agricultural genetics and animal breeding. They are designed for genetic analysis of continuously varying quantitative traits such as body mass index (BMI), cholesterol levels, or intelligence quotient. They can be used to assess the strength of genetic effects on a trait, to localize genes influencing a trait through either linkage or association methods, to assess whether associated variants are likely to be the functional variants behind a given localization signal, to explore whether related traits have shared genetic influences in multivariate analyses, and to characterize the genetic effects on a trait through analyses of gene-gene and gene-environment interactions.