1. Department of Agricultural and Environmental Biology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
2. Maebashi Institute of Animal Science, Livestock Improvement Association of Japan, Inc., Maebashi 371-0121, Japan
3. Agricultural Research Center, Ipponmatsu, Torigoe, Shinjo, Yamagata 996-0041, Japan
4. Cattle Breeding Department (beef), Livestock Improvement Association of Japan, Inc., Tokyo 135-0041, Japan