POWER OF TESTS FOR QTL DETECTION USING REPLICATED PROGENIES DERIVED FROM A DIALLEL CROSS

In crop species, most QTL (quantitative trait loci) mapping strategies use segregating populations derived from an initial cross between two lines. However, schemes including more than two parents could also be used. We propose an approach using a high-density restriction fragmen t length polymorphism (RFLP) map established on six F2 populations der ived from diallel crosses among four inbred lines and the phenotypic p erformances of two types of replicated progenies (F3 and topcross). Th e QTL is supposed to be on the marker locus considered. Three linear m odel tests for the detection of QTL effects (T1, T2 and T3) are descri bed and their power studied for the two types of progeny. T1 tests the global genetic effects of the QTL (additivity and dominance) and T2 t ests only additive effects assuming dominance is absent when it could exist. The models of these two tests assume that the main effects of Q TL alleles are constant in different genetic backgrounds. The additive model of test T3 considers the six F2 populations independently, and T3 is the equivalent of the classical mean comparison test if we negle ct dominance; it uses only contrasts between the homozygote marker cla sses. The results show that T2 is much more powerful than T3. The powe r of T1 and T2 depends on the relative sizes of the additive and domin ance effects, and their comparison is not easy to establish. Neverthel ess, T2 seems to be the more powerful in most situations, indicating t hat it is often more interesting to ignore dominance when testing for a QTL effect. For a given size of genetic effects, the power is affect ed by the total number of individuals genotyped in F2 and the recombin ation rate between the marker locus and the putative QTL. The approach presented in this paper has some drawbacks but could be easily genera lized to other sizes of diallels and different progeny types.