J. A. S., J. A. P. and A. E. have been funded by the Spanish Ministries of Agriculture Fisheries and Food (Jacumar Project, Ref.: 96102102). The Authors thank Woolliams J. (Roslin Institute) for his suggestions and help with the effective breeding number estimations in this work and two anonymous referees who gave useful comments about the manuscript. Phil Hedges (UK) helped with language corrections.