Behavior Genetics: From Heritability to Gene Finding

Abstract

INTRODUCTION The aim of this chapter is to review and discuss the designs and methods of behavior genetics and genetic epidemiology. We define these fields broadly as the study of the role of genes and environment in phenotypic individual differences, where the term phenotype is employed for any measured (observed) characteristic or trait, i.e., ranging from biomarkers in blood, MRI or EEG data, and blood pressure, to personality traits, psychiatric disorders, and lifestyle. While behavior genetics often is associated with psychology, we define it to encompass any human phenotype which is measurable and can be characterized by appreciable individual differences. We limit this chapter to human behavior genetics, but acknowledge the common foundation of animal and human genetics in the biometrical model (Fisher, 1918; Falconer & Mackay, 1996; Lynch & Walsh, 1998; Plomin, DeFries, Knopik, & Neiderhiser, 2013) and quantitative genetic theory. Individual differences, as far as they are inherited, often depend on genes with on average small effects and are usually influenced by many genetic variants. The methods of quantitative genetics are characterized by studies of large populations and families/twins and by the fact that the nature of quantitative differences requires the measurement (and not just the classification) of individuals. Quantitative genetic theory is concerned with the genetic properties of populations (population genetics) and the inheritance of measurements (biometrical genetics) and is now merging with molecular genetics with the realistic aim of localization and characterization of genes causing quantitative variation. We focus on human phenotypes that are complex, i.e., subject to the influence of many genes and environmental exposures. Before 1990 behavior genetics was dominated by family, adoption, and twin studies, in which the contributions of unmeasured (or “latent”) genetic and environmental factors to phenotypic variation were inferred from the phenotypic resemblance among family members (Martin & Eaves, 1977; Posthuma et al., 2003). This work established beyond reasonable doubt the importance of genetic and environmental factors in a variety of human phenotypes (McGue & Bouchard Jr., 1998; Bouchard & McGue, 2003; Polderman et al., 2015) and set the stage for addressing follow-up issues, including questions about comorbidity and multivariate traits, the analyses of longitudinal data and time-series, the interaction and covariance of genes and environment, and the identification of genetic variants at the DNA sequence level.