QUANTITATIVE TRAIT LOCI ANALYSIS FOR 5 MILK-PRODUCTION TRAITS ON CHROMOSOME-6 IN THE DUTCH HOLSTEIN-FRIESIAN POPULATION

Twenty Dutch Holstein-Friesian families, with a total of 715 sires, we re evaluated in a granddaughter experiment design for marker-QTL assoc iations. Five traits-milk, fat and protein yield and fat and protein p ercent-were analyzed. Across-family analysis was undertaken using mult imarker regression principles. One and two QTL models were fitted. Cri tical values for the test statistic were calculated empirically by per muting the data. Individual trait distributions of permuted test stati stics differed and, thus distributions, had to be calculated for each trait. Experimentwise critical values, which account for evaluating ma rker-QTL associations on all 29 autosomal bovine chromosomes and for f ive traits, were calculated. A QTL for protein percent was identified in one and two QTL models and was significant at the 1 and 2% level, r espectively. Extending the multimarker regression approach to an analy sis including two QTL was limited by families not being informative at all markers, which resulted in singularity. Below average heterozygos ity for the first and last marker lowered information content for the first and last marker bracket. Highly informative markers at the ends of the mapped chromosome would overcome the decrease in information co ntent in the first and last marker bracket and singularity for the two QTL model.