1. College of Animal Science and Technology; Northwest A&F University; Yangling Shaanxi 712100 China
2. School of Life Science and Technology; ShanghaiTech University; Shanghai 201210 China
3. Institute of Farm Animal Genetics; Friedrich-Loeffler-Institut; Neustadt 31535 Germany
4. Ningxia Tianyuan Sheep Farm; Hongsibu 751999 China