Seed dormancy is a key adaptive trait under polygenic control in many plants. We introduced the chromosomal regions containing the dormancy QTLs qSD1, qSD7-1, and qSD12 from an accession of weedy rice into a nondormant genetic background to examine component genetic effects and their interactions with time of afterripening (DAR). A BC 4 F 2 plant, which was heterozygous for the three loci, was selected to develop the BC 4 F 3 population. Single point analysis detected only qSD7-1 and qSD12 (R 2 ¼ 38-72%) at 10, 30, and 50 DAR in the population. However, multiple linear regression analysis detected genetic effects of the three QTLs and their trigenic epistasis, an environmental effect of DAR (E), and interactions of E with qSD12 and with the qSD1 Â qSD7-1 and qSD7-1 Â qSD12 epistases. The linear model demonstrates that QTL main effects varied with DAR, and that some epistasis or epistasis-by-DAR interactions partially counteract the main effects. The three QTLs were isolated as single Mendelian factors from the BC 4 F 3 population and estimated for component genic effects based on the BC 4 F 4 populations. Isolation improved estimation of the qSD1 effect and confirmed the major effect of qSD12. The qSD1 and qSD12 loci displayed a gene-additive effect. The qSD7-1, which was further narrowed to a chromosomal region encompassing the red pericarp color gene Rc, displayed gene additive and dominant effects. Heredity (2006) 96, 93-99.