1. Colegio de Postgraduados, Montecillo, Texcoco 56230, México
2. Departments of Animal Sciences, Dairy Science, and Biostatistics and Medical Informatics, University of Wisconsin-Madison, Madison, Wisconsin 53706
3. Biometrics and Statistics Unit and Global Wheat Program, International Maize and Wheat Improvement Center (CIMMYT), 06600 Mexico, D.F., México