QTL Analysis in Plants

Quantitative traits are defined as traits that have a continuous phenotypic distribution (1 ,2 ). Variances of these traits are often controlled by the segregation of many loci, called quantitative trait loci (QTL). Therefore, quantitative traits are often synonymously called polygenic traits. Another characteristic of quantitative traits is that environmental variates can play a large role in determining the phenotypic variance. The polygenic nature and the ability of being modified by the environment make the study of genetic basis for quantitative traits more difficult than that for monogenic traits. Traditional methods of quantitative genetics that use only the phenotypic and pedigree information cannot separate the effects of individual loci but examine the collective effect of all QTL. With the rapid development of molecular technology, a large number of molecular markers (DNA variants) can be generated with ease. Most molecular markers are functionally neutral, but they normally obey the laws of Mendelian inheritance. Therefore, the relative positions of the markers along the genome (called the marker map) can be reconstructed using observed recombin ant events. The joint segregating patterns of markers, in conjunction with phenotypic and pedigree information, provides additional information about the genetic basis of quantitative traits, including the number and chromosomal locations of QTL, the mode of gene action, and sizes (effects) of individual QTL. A complete description of the properties of QTL is called the genetic architecture. The study of the genetic architecture of quantitative traits using molecular markers is called QTL mapping.