| DNA markers such as microsatellite markers have made it possible to identify genes at the molecular level for quantitative traits, which are complex in nature. The numbers of genes and the effects of individual genes that control quantitative traits are unknown. The overall objective of this thesis was to identify quantitative trait loci (QTL) affecting milk production traits and to estimate their effects on chromosomes one, six, nine, ten and twenty in Canadian Holstein bulls.; A granddaughter design with 6 grandsires and a total of 432 sons was used in this study. About seven markers were genotyped in each chromosome. Markers were mapped using ANIMAP and CRI-MAP programs. Least squares (LS) and restricted maximum likelihood (REML) methods were employed to detect QTL and to estimate their effects or variance. To identify significant QTL, empirical threshold values based on a permutation method, and χ2 table values were used in LS and REML methods, respectively.; A single QTL at about 55 cM involved in the control of both milk and protein yield, and another QTL affecting fat yield and fat% were identified around 32 cM on chromosome one. A single QTL at about 50 cM for both milk and protein yield, and another for protein% around 13 cM were detected on chromosome six. A single QTL possibly affecting all yield traits around 10 cM and another QTL affecting protein% around 64 cM were identified on chromosome twenty. Preliminary analysis showed evidence for presence of QTL for fat% and protein% on chromosome ten.; The impact on detection of QTL and estimating their effects using alternative trait values, daughter yield deviation (DYD), estimated breeding value (EBV) and deregressed proofs (DRP), was investigated. Similar results were observed regardless of the trait values used. The effects of using the estimated marker map from this study versus a published marker map on detection and estimation of QTL effect was also investigated. In terms of significant evidence found and of relative positions of identified QTL, results were similar in both maps. The identified QTL in the individual grandsires could be utilized in animal breeding programs. |
