Mapping of quantitative trait loci by using genetic markers: an overview of biometrical models used

In crop plants quantitative variation is a feature of many important traits, such as yield, quality or disease resistance. Means of analyzing quantitative variation and especially of uncovering its potential genetic basis are therefore of prime importance for breeding purposes. It has been demonstrated in the early 20th century that such quantitative variation results from the combined action of multiple segregating genes and environmental factors. An intrinsic feature of such traits is, however, that the individual genes contributing to quantitative variation can hardly be distinguished. The genetics of such complex traits is therefore studied in general terms (population means and variances, covariances between progenies, heritabilities and so on) of classical quantitative genetics, rather than in terms of individual gene effects. Only by the use of genetically marked chromosomes, is it possible to detect and locate the loci affecting quantitative traits ("quantitative trait loci" or "QTLs"). Linkage between QTLs and morphological markers has been reported, but accurate and systematic genetic mapping has been hampered by the lack of a sufficient number of genetic markers covering an entire genome. Recently, new tooh have become available by the advent of molecular markers, such as restriction fragment length polymorphisms (RFLPs). Now, dense genetic linkage maps exist for many plant and animal species, which heralds a new era for quantitative genetics. Powerful and accurate biometrical methods are needed, so as to make possible the dissection of quantitative variation of complex characters into individual QTL effects. Mapped QTLs can be traced in breeding programmes, for instance, indirectly by selection for linked markers, or they can be cloned and introgressed via molecular or cell-biological techniques. The traditional methods for mapping of QTLs are, however, neither powerful nor accurate and the development of better methods is an area open to research. Not surprisingly, the detection and mapping of QTLs is gaining rapidly growing attention from biometrical geneticists.