Many QTLs with minor additive effects are associated with a large difference in growth between two selection lines in chickens

• Two growth-selected lines in chickens have been developed from a single founder population bydivergent selection for body weight at 56 days of age. After more than 40 generations of selectionthey show a nine-fold difference in body weight at selection age and large differences in growth rate,appetite, fat deposition and metabolic characteristics. We have generated a large intercross betweenthese lines comprising more than 800 F2 birds. QTL mapping revealed 13 loci affecting growth.The most striking observation was that the allele in the high weight line in all cases was associatedwith enhanced growth, but each locus explained only a small proportion of the phenotypic varianceusing a standard QTL 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 the red junglefowl andWhite Leghorn domestic chickens is explained by a small number of QTLs with large additiveeffects. Furthermore, no QTLs for anorexia or antibody response were detected despite largedifferences for these traits between the founder lines. The result is an excellent example where alarge phenotypic difference between populations occurs in the apparent absence of any single locuswith large phenotypic effects. The study underscores the need for powerful experimental designs ingenetic studies of multifactorial traits. No QTL at all would have reached genome-wide significanceusing a less powerful design (e.g. approx. 200 F2 individuals) regardless of the nine-fold phenotypicdifference between the founder lines for the selected trait.

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