1. Institute of Animal Genetics and Breeding, College of Animal Science and Technology, Sichuan Agricultural University, Chengdu 611130, China
2. Animal Breeding and Genetics Key Laboratory of Sichuan Province, Pig Science Institute, Sichuan Animal Science Academy, Chengdu 610066, China
3. Division of Genetics, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, 02115 USA
4. Yak Research Institute, Sichuan Academy of Grassland Science, Chengdu 610097, China
5. Department of Animal Science, Texas A & M University, College Station, Texas, 77843 USA