Mapping quantitative trait loci (QTL) within commercial populations circumvents the need to verify segregation of QTL detected under experimental crosses. Variance component (VC) mapping combines information from all relationships within a pedigree to provide a breeding value for the animal and QTL with immediate potential for marker assisted selection. Variance components are estimated by modelling the covariance structure between relatives sharing alleles identical by descent (IBD). Commercial poultry data and simulation will be used as a model to explore how VC methodology behaves under varying population structures. Data is available from the Cobb Vantress broiler population to evaluate extensions to the additive QTL model. Computer simulation will be used to test models including additive, dominance, epistatic and imprinting effects over a range of pedigree structures including family sizes and number of generations.