Epsilon Open Archive

Abstract

The aims of this thesis were to improve the knowledge about the genetic background of health and reproduction traits in dairy cattle. This was accomplished by performing gene mapping studies of quantitative trait loci (QTL) affecting these traits and also by exploring the genetic variability for pathogen-specific mastitis. Ten sire families were used in a granddaughter design to map QTL. Several QTL were detected for most of the traits studied. Inclusion of cofactors in the analyses to adjust for QTL found on other chromosomes increased both the number of QTL detected and the significance levels. In total, 20 QTL were detected in analyses without cofactors whereas 41 QTL were detected when cofactors were used. Chromosome regions with several QTL were found, e.g. on chromosome 9 and 11 we observed QTL for both health and reproduction traits. A QTL for resistance to mastitis was found close to the same marker where a QTL for the fertility measure non-return rate was detected. The QTL affecting non-return rate was fine mapped to a small region on chromosome 9. Further research is required to evaluate whether this QTL could be used in breeding programs by marker assisted selection. Resistance to mastitis is a multifactorial trait that involves a variety of pathogens and physiological defense mechanisms. To dissect the trait into sub-traits of resistance to individual pathogens, bacteriological data on mastitis pathogens were analyzed. The most common pathogen in the data set was Staphylococcus aureus. Genetic variation in resistance to individual mastitis pathogens was found. Estimated heritabilities for the sub-traits were higher compared to heritabilities for general resistance to mastitis in most other studies. Haplotypes of a QTL previously shown to affect resistance to mastitis were analyzed for effects on resistance to pathogen specific mastitis. No significant allele substitution effect of haplotype on occurrence of the various mastitis pathogens was found, but there was a significant difference between the effects of two of the haplotypes on the presence of Streptococcus dysgalactiae.