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

Departments of Dairy Science and Statistics, Virginia Tech, Blacksburg, VA 24061-0315 USA

Quantitative trait loci 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 2,497 autosomal cM using Kosambi's mapping function. All families were analyzed jointly with least-squares (LS) and variance components (VC) methods. A total of 6 QTLs exceeding approximate experiment -wise significance thresholds, 24 QTLs exceeding suggestive thresholds, and 34 QTLs exceeding 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 interval for the experiment-wise significant QTLs was 48cM. Some chromosomes harbored QTLs affecting several traits, and these were always in coupling phase, defined by consistency with genetic correlations among traits. Chromosome 17 likely harbors two QTLs affecting milk yield, and some other chromosomes showed some evidence for two linked QTLs affecting the same trait. In each of these cases , the two QTLs were in repulsion phase in those families appearing to be heterozygous for both QTLs, a finding which supports the build-up of linkage disequilibrium due to selection.