MAPPING QUANTITATIVE TRAIT LOCI FOR MILK-PRODUCTION AND HEALTH OF DAIRY-CATTLE IN A LARGE OUTBRED PEDIGREE

Quantitative trait loci (QTL) affecting milk production and health of dairy cattle were mapped in a very large Holstein granddaughter design . The analysis included 1794 sons of 14 sires and 206 genetic markers distributed across all 29 autosomes and flanking an estimated 2497 aut osomal cM using KOSAMBI's mapping function. All families were analyzed jointly with least-squares (LS) and variance components (VC) methods. A total of 6 QTL exceeding approximate experiment-wise significance t hresholds, 24 QTL exceeding suggestive thresholds, and 34 QTL exceedin g chromosome-wise thresholds were identified. Significance thresholds were determined via data permutation (for LS analysis) and chi-square distribution (for VC analysis). The average bootstrap confidence inter val for the experiment-wise significant QTL was 48 cM. Some chromosome s harbored QTL affecting several traits, and these were always in coup ling phase, defined by consistency with genetic correlations among tra its. Chromosome 17 likely harbors 2 QTL affecting milk yield, and some other chromosomes showed some evidence for 2 linked QTL affecting the same trait. In each of these cases, the 2 QTL were in repulsion phase in those families appearing to be heterozygous for both QTL, a findin g which supports the build-up of linkage disequilibrium due to selecti on.