Our dearth of knowledge about quantitative variation is readily apparent in our still poor understanding of the majority of multigenic diseases affecting humans. While the past decade has seen dramatic progress in the development of computational approaches to study complex traits, a molecular understanding of the basis of natural variation has remained elusive. Systems in which QTL genes are known offer exciting opportunities to determine the types of molecular changes that lead to variation in populations. The proposed work will characterize a known QTL gene, teosinte branched-i (tbi). Differences in the regulation of the tbi gene account for much of the variation in morphology between domesticated maize and its wild ancestor, teosinte. Detailed expression studies will be used to correlate differences in tbi transcription with developmental outcomes, and sequence diversity studies will be used to identify selected regions of the tbi locus responsible for derived maize morphology. Finally, tbi genic regions that underlie differences in maize-teosinte morphology will be functionally localized in a 10,000 plant cross and characterized. Understanding how the tbi QTL gene contributes to quantitative variation will be of broad interest to researchers examining complex traits in many organisms including humans.