1. Colegio de Postgraduados, Km. 36.5 Carretera Mexico-Texcoco, C.P. 56230
2. Department of Animal Sciences, University of Wisconsin, Madison, 53706
3. Department of Dairy Science, University of Wisconsin, Madison, 53706
4. Department of Biostatistics and Medical Informatics, University of Wisconsin, Madison, 53706
5. Biometrics and Statistics Unit, International Maize and Wheat Improvement Center (CIMMYT), Apdo. Postal 6-641, Mexico D.F., Mexico