1. Research Institute for the Biology of Farm Animals (FBN), Research Group Functional Genomics, Wilhelm-Stahl-Allee 2, 18196 Dummerstorf, Germany, Research Institute for the Biology of Farm Animals (FBN), Research Unit Molecular Biology, Wilhelm-Stahl-Allee 2, 18196 Dummerstorf, Germany, and Animal Breeding and Husbandry Group, Institute of Animal Science, University of Bonn, Endenicher Allee 15, 53115 Bonn, Germany