1993. Identification of quantitative trait loci (QTL) in maize on the basis of F3 and testcross progeny performance

1993. Identification of quantitative trait loci (QTL) in maize on the basis of F3 and testcross progeny performance

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INTRODUCTION

The phenotypic expression of a quantitative trait is due to the combined genetic effects of several to many genes, each contributing a small amount to total phenotype, plus environmental effects (Falconer, 1981). Quantitative genetic theory is based on the idea that genes controlling quantitative characters are subject to the same laws of inheritance as those controlling qualitative characters. In practice, however, quantitative traits are usually described in biometrical terms and gene effects are not considered individually. One way to study a quantitative trait at the gene level is to identify associations between phenotypic expression of the trait and the genotypes of scorable markers, resulting from linkage between the marker loci and quantitative trait loci (QTL).

Until recently, the ability to identify marker-QTL linkages has been hindered by the lack of marker systems that could provide plentiful informative markers in germplasm of interest (Sax, 1923, Thoday, 1961). This limitation has been largely overcome by the development of molecular marker systems, such as RFLPs, which can detect polymorphisms at the DNA sequence level. Such markers have been used to construct linkage maps in many major crop species (Tanksley et al., 1989; Melchinger, 1990) permitting QTL mapping throughout all or much of the genome. QTL analysis has also benefited from advances in statistical techniques for detecting associations between marker genotypes and phenotypic effects. Maximum likelihood and least squares methods have been developed to conduct searches for QTL in intervals between linked markers (Lander and Botstein, 1989; Knapp et al., 1990; Haley and Knott, 1992).

While these advances have greatly improved the ability to estimate the positions and effects of QTL, the actual expression of QTL effects remains dependent on characteristics of the population and how it is evaluated phenotypically. QTL analysis, like biometrical analysis of quantitative traits, is subject to factors, such as, the germplasm evaluated (Abler et al., 1991; Beavis, et al., 1991) whether individuals or their progenies are evaluated (Cowen, 1988; Soller and Beckman, 1990), and environmental conditions (Patterson et al., 1991)  .