For monogenic traits, dominance is largely straightforward: How a trait manifests (its phenotype) is governed by how many copies of a dominant and/or recessive allele one inherits. The picture for polygenic traits, driven by loci throughout the genome, is less clear: In what way do individual variants influence phenotype? To explore this question, Ben Neale and colleagues developed an approach called dominance LD score regression (d-LDSC) and applied it to UK Biobank data on 1,060 traits. They found that dosage-dependent additive effects accounted for the vast majority of heritable trait variation in this population.