Quantitative trait loci (QTLs) have been mapped in many studies of F2populations derived from crosses between diverse lines. One approach toconfirming these effects and improving the mapping resolution is geneticchromosome dissection through a backcrossing programme. Analysis by intervalmapping of the data generated is likely to provide additional power andresolution compared with treating data marker by marker. However, intervalmapping approaches for such a programme are not well developed, especially wherethe founder lines were outbred. We explore alternative approaches to analysisusing, as an example, data from chromosome 4 in an intercross between wild boarand Large White pigs where QTLs have been previously identified. A least squaresinterval mapping procedure was used to study growth rate and carcass traits in asubsequent second backcross generation (BC2). This procedure requires theprobability of inheriting a wild boar allele for each BC2 animal for locationsthroughout the chromosome. Two methods for obtaining these probabilities werecompared: stochastic or deterministic. The two methods gave similarprobabilities for inheriting wild boar alleles and, hence, gave very similarresults from the QTL analysis. The deterministic approach has the advantage ofbeing much faster to run but requires specialized software. A QTL for fatnessand for growth were confirmed and, in addition, a QTL for piglet growth fromweaning at 5 weeks up to 7 weeks of age and another for carcass length weredetected.