Date: 30 Jul 2012

Basic Concepts of Quantitative Genetics

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Abstract

Quantitative genetics is a special branch of genetics, which is concerned with the inheritance of the differences between individuals that are measured in degree rather than in kind. These individual differences are referred to as quantitative differences orquantitative traits. Formally, a quantitative trait is defined as a trait whose value varies continuously across individuals (Falconer and Mackay 1996; Lynch and Walsh 1998). The phenotype of a quantitative trait measured from an individual is not determined by genes alone; it is also determined by environmental variants. The proportion of the phenotypic variance explained by the segregation of a single gene is usually small. However, the contribution of all these small-effect genes collectively is significant to the variation of the phenotype. Genes controlling the variation of a quantitative trait are calledquantitative trait loci (QTL). Note that the term QTL defined in this book is used for both the singular and plural forms, e.g., one QTL for weight and two QTL for height. In the quantitative genetics literature, QTL represents the singular form and QTLs is used as the plural form. No matter how small a QTL is, it segregates just like a regularMendelian locus. For small-effect QTL, we simply cannot observe the segregation and must resort to statistical methods to infer the segregation. Most statistical methods applied in quantitative genetics require specific genetic models, which will be the focus of this chapter.