The Roslin Institute, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Midlothian, Scotland, UK. trevor.paterson@roslin.ed.ac.uk|The Roslin Institute, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Midlothian, Scotland, UK. trevor.paterson@roslin.ed.ac.uk|The Roslin Institute, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Midlothian, Scotland, UK. trevor.paterson@roslin.ed.ac.uk|The Roslin Institute, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush, Midlothian, Scotland, UK. trevor.paterson@roslin.ed.ac.uk