DETECTION, NUMBER AND EFFECTS OF QTLS FOR A COMPLEX CHARACTER

Factors influencing the detection of quantitative trait loci (QTLs) via molecular markers are discussed in the case of recombinant inbred lines. The importance of the residual genetic variance present is emphasized and an expression for the minimum detectable effect is presented, which only depends on the design (trait heritability and number of lines studied); for relatively realistic situations, this minimum effect may be as high as 20 to 30% of the genetic SD. Therefore only QTLs with relatively large effects, and that are sufficiently closely (inked to the marker, are likely to be detected. Increasing the genetic distance between parents can increase the probability of finding crosses in which significant tie large-effect) QTLs are found, at the expense of decreasing the probability of detecting existing small-effect QTLs. Epistasis can diminish QTL detectability. its possible importance has been discussed, and appears to be difficult to demonstrate. it has been shown that for a complex character, detected QTLs can at most only constitute a small subset of the existing QTLs. Furthermore, the occurrence of numerous small-effect QTLs regularly distributed over a chromosomal fraction in a directional manner has been shown to lead to the artefactual detection of a QTL by a centrally located marker.