25Heterosis has been key to the development of maize breeding but describing its genetic basis has 26 been challenging. Previous studies of heterosis have shown the contribution of within-locus 27 complementation effects (dominance) and their differential importance across genomic regions. 28 However, they have generally considered panels of limited genetic diversity and have shown 29 little benefit to including dominance effects for predicting genotypic value in breeding 30 populations. This study examined within-locus complementation and enrichment of genetic 31 effects by functional classes in maize. We based our analyses on a diverse panel of inbred lines 32 crossed with two testers representative of the major heterotic groups in the United States (1,106 33 hybrids), as well as a collection of 24 biparental populations crossed with a single tester (1,640 34 hybrids). We assayed three agronomic traits: days to silking (DTS), plant height (PH) and grain 35 yield (GY). Our results point to the presence of dominance for all traits, but also among-locus 36 complementation (epistasis) for DTS and genotype-by-environment interactions for GY. 37Consistently, dominance improved genomic prediction for PH only. In addition, we assessed 38 enrichment of genetic effects in classes defined by genic regions (gene annotation), structural 39 features (recombination rate and chromatin openness), and evolutionary features (minor allele 40 frequency and evolutionary constraint). We found support for enrichment in genic regions and 41 subsequent improvement of genomic prediction for all traits. Our results point to mechanisms by 42 which heterosis arises through local complementation in proximal gene regions and suggest the 43