Two growth-selected lines in chickens have been developed from a single founderpopulation by divergent selection for body weight at 56 days of age. After morethan 40 generations of selection they show a nine-fold difference in body weightat selection age and large differences in growth rate, appetite, fat depositionand metabolic characteristics. We have generated a large intercross betweenthese lines comprising more than 800 F2 birds. QTL mapping revealed 13 lociaffecting growth. The most striking observation was that the allele in the highweight line in all cases was associated with enhanced growth, but each locusexplained only a small proportion of the phenotypic variance using a standardQTL model (1.3-3.1%). This result is in sharp contrast to our previous studywhere we reported that the two-fold difference in adult body size between thered junglefowl and White Leghorn domestic chickens is explained by a smallnumber of QTLs with large additive effects. Furthermore, no QTLs for anorexia orantibody response were detected despite large differences for these traitsbetween the founder lines. The result is an excellent example where a largephenotypic difference between populations occurs in the apparent absence of anysingle locus with large phenotypic effects. The study underscores the need forpowerful experimental designs in genetic studies of multifactorial traits. NoQTL at all would have reached genome-wide significance using a less powerfuldesign (e.g. approx. 200 F2 individuals) regardless of the nine-fold phenotypicdifference between the founder lines for the selected trait.