Identification and examination of molecular genetic markers associated with somatic cell scores of Ontario Holstein cattle
The main purpose of this research was to identify molecular genetic markers associated with somatic cell scores (SCS) for genetic improvement of mastitis resistance in Ontario Holsteins. Mastitis resistance is expected to be improved via selection based on the molecular genetic markers associated with SCS. The statistical power of detecting QTL-marker linkage via bulked segregant analysis is a function of the size of the segregating population, the proportion selected in the extreme tails of the phenotypic distribution, the size of gene effects, and the degree of dominance of the QTL. By increasing the size of the recorded population and decreasing the proportion selected, the number of DNA extractions to form the DNA bulks and genotyping reactions can be substantially decreased for a given statistical power. Bayesian segregation analysis using Gibbs sampling approach was applied for analyzing a set of field data for SCS and for comparison four sets of data simulated with different genetic parameters. The segregation analysis of field data for SCS suggested that occurrence of a major gene significantly affected the SCS of Ontario Holsteins. The estimated heritability of SCS was approximately 0.16. The major gene variance accounted for about 17% of the total genetic variance and the estimate of the frequency of the positive allele was 0.30. However, the precision of these estimates was questionable, based on the results of simulation, and the actual QTL effect seemed likely to be underestimated. Bulked segregant analysis was applied to detect amplified fragment length polymorphism (AFLP) associated with SCS. Seventy primer pair combinations from eight 'Eco'R I and nine 'Taq' I primers were used to screen the genomes of Ontario Holsteins. Only fragments smaller than 500bp were screened using the ABI 377 sequencer with the internal size standard GS-500 ROX. Two AFLP fragments (151 bp and 214 bp) were found to be much more frequent among cows with high estimated breeding value (EBV) for SCS and two other AFLP fragments (105 bp and 261 bp) were much more prevalent among cows with low EBV for SCS. Those markers can potentially be used to select for increased mastitis resistance.