The primary goal of this study was to investigate statistical properties of amixed inheritance model for the localization of quantitative trait loci (QTL).This is based on the analysis of phenotypic data for the amount of intramuscularfat (IMF) scored on 305 individuals originating from a cross between Duroc andNorwegian Landrace breeds. Marker genotype information is available for F1 andF2 generations. Statistical procedures compared involve i) the interval mapping,ii) the composite interval mapping, iii) a regression method, and iv) a mixedinheritance model accounting for a random animal additive genetic effect andrelationships between individuals. The basic statistical properties of thelatter approach are then assessed using Monte Carlo simulations showing slightunconservativeness as compared to chi(2)2df and reasonable power to detect QTLof moderate effects. In the analysis of IMF data, the significant evidence forthe existing QTL is detected on chromosome 6. A chromosomal region recommendedfor a second-step fine mapping analysis is identified between markers SW1823 andS0228, based on three types of confidence intervals derived by using: i) theJackknife algorithm, ii) the numerical variance approximation, and iii) the LODscore approach. The Jackknife algorithm was additionally used to quantify eachfamily's contribution to the test statistic and to the estimate of QTL position.