Quantitative trait loci (QTLs) with large effects on fatness and growth havebeen identified previously on pig chromosome 4 in an intercross between theEuropean wild pig and Large White domestic pigs. Two F2 sows, heterozygous forthe actual chromosome region, were backcrossed to a Large White boar, and twobackcross (BC1) boars were in turn backcrossed to Large White/Landrace sows. Oneof the boars was heterozygous for an intact wild pig-derived QTL region, whereasthe other carried a recombinant haplotype. A total of 85 BC2 animals wereproduced. Phenotypical measurements included daily weight gain, ultrasonicmeasurements of fat depth at 70 and 90 kg and several carcass traits. QTLsegregation was deduced using 15 markers previously assigned to chromosome 4.Highly significant QTL effects were observed on all fatness traits and on thelength of the carcass. A small but significant effect on growth was alsoobserved. The results confirm the presence of one or more QTLs on chromosome 4affecting fatness and growth. There was a good agreement between the estimatesof QTL effects in the F2 and BC2 generations. The results from the recombinantsire family allowed us to map the major QTL effect distal to the recombinationbreakpoint. We propose that this confirmed QTL with a major effect on fatness isdesignated FAT1.