A mapped QTL defines the location of a gene (or more than one gene) that influences a complex trait (i.e., one that is influenced by multiple genes, by environmental factors and can be influenced by all possible interactions of these variables). The principles and complexities of QTL mapping, and the importance of a systems genetics approach, have been recently presented in an excellent review (Mackay et al. 2009). Gerald McClearn was among the first to consider gene–gene interaction effects on alcohol preference drinking in mice as one of the complicating factors in identifying genetic influences (Fernandez et al. 2000). QTL mapping results for murine ethanol consumption were examined for replicability using meta-analysis in 2001 (Belknap and Atkins 2001). The analysis included only QTL studies that used populations derived from the C57BL/6J and DBA/2J inbred strains. These strains are typically at the top and bottom, respectively, of lists comparing inbred strains for voluntary alcohol consumption/preference (Belknap et al. 1993; Wahlsten et al. 2006; Yoneyama et al. 2008). In all 9 studies, data were available that compared the consumption of 10% ethanol
