Quantitative Genetics

Genetic analysis of quantitative traits is applied in a wide variety of substantive research areas. Many human characteristics exhibit continuous variation, such as height, weight, intelligence or personality, but the application of quantitative genetics is not limited to traits thought to be continuous. For example, eye color may be assessed in a qualitative fashion – blue versus brown versus green – but there exists substantial variation within these categories that could be measured and therefore quantified. Even apparently binary characteristics, such as having had heart disease or not, may best be understood in terms of an underlying continuum of predisposition to heart disease, rather than as a qualitative outcome. Conversely, a truly qualitative, monogenic trait could be considered as a quantitative trait that has a single source of variation. Therefore, almost all genetics is, at its base, concerned with quantitative traits. Comprehension of the basic elements of a quantitative genetic model requires some understanding of how we measure variation, or individual differences.