| The study and discussion largely focused on three aspects as follows in this paper:1)Selection may be ubiquitous in experimental populations when we detect the presence of QTL through the method of linkage analysis. The existence of selection will bring about biases for some respects of the analysis. Thus, it is to some extent of practical significance that how we decide upon the effect of selection on the linkage analysis in experimental populations .In this paper, based upon the precondition of half-sib design and flanking genetic markers, the change of statistical power for detecting QTL was investigated as selection intensity varied. Several concrete forms of variance components were given, at the same time also given were regular changes varying with different selection intensities of these variance components. The concept of relative number (RN) was put forward by which the statistical power for QTh detection was accounted for under different selection intensities.With this inerratic change for the power of QTL detection,suitable experimental sample size could be roughly determined in practice pursuant to the degree of selection in the population of interest prior to experiment in order that the statistical power for QTh mapping would not decline because of the presence of selection in the population. Due to the fact that the presence of selection could result in biases for estimates of QTL linkage analysis and effect estimating, these kinds of biases should be considered and manage to avoid them when carrying on the linkage analysis of marker-QTL by use of selected data, otherwise the reliability of results will decrease. These biases fundamentally were caused by the Image disequilibria both between QTL genotypes and polygenes and between QTL and genetic markers.2)At present the practice of QTL mapping has found out some major loci that are of breeding significance. How on earth do we put the information of these major loci into breeding scheme? Compared with conventional selection systems, whether or not theselection that utilizes the information of identified major loci is to better advantage? In this paper, the relative selection efficiency (RSE) for direct selection of QTL (DSQ) was discussed subject to the mixed genetic model that is a single identified locus plus polygenes, and to the breeding objective that includes two correlative traits, some factors which may act a part on the RSE were synchronously examined to the effect that we could sort out some vital ones which would be brought into flu! play in practical applications. Such factors as proportion of additive genetic variance due to major loci, heritability, correlation between traits, gene frequency on major loci, major genotypic value, dominant genotypic value within major loci and substitution effect of major loci were mainly considered. These factors themselves had different influences upon the RSE of DSQ under the two-trait breeding goal, and the mathematical models that rendered the RSE of DSQ fluctuate mutually differed. But taken one with another, these elements like the proportion of additive genetic variance explained by major Ioci, heritability, substitution effect and major loci genotypic value relatively took on greater effect on the RSE of DSQ. Results showed that we could gain larger benefits from the use of major loci in selection scheme as long as we rationally introduce the information of major loci concerned into selection scenario. From the RSE point of view, DSQ can indeed bring about additional genetic gain.3)Advances in molecular genetics currently have raised the possibility of direct selection on the genotypes for genetic improvement in principal animal traits of economic interest. However, what breeders are asking is how much advantage this new technology will justifiably offer over existing technology from genotypic analysis for animals. In especial whether or not this advantage will keep on in the course of execution of multigeneration selection scenario. The RSE of continuous multigeneration selec...
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