1. Department of Biostatistics, School of Public Health, University of Alabama, Birmingham, Alabama 35294
2. School of Environmental and Rural Science, University of New England, Armidale 2351, New South Wales, Australia
3. The Roslin Institute, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Midlothian, EH25 9RG, Scotland
4. Biosciences Research Division, Department of Primary Industries, Bundoora 3083, Victoria, Australia
5. Animal Breeding and Genomics Centre, Wageningen UR Livestock Research, 8200 AB Lelystad, The Netherlands