Details regarding the cross between WL males and RIR females and the F2 population thus derived were described by Sasaki et al. (2004). In brief, F1 chickens were produced by crossing 5 WL males with 16 RIR females, with 1 to 4 females randomly selected to mate with each male. We obtained 272 F2 females by crossing 16 F1 males with 60 F1 females, with 1 to 5 full-sib females randomly selected to mate with each male. The F2 chickens were hatched on the same day in the same incubator. These birds were raised in the same chicken house and fed the same food for the duration of the experiment. Experiments were performed according to the guidelines for the care and use of agricultural animals in agricultural research and teaching.