For many monogenic traits, the genetic variants that underpin those traits act in dominant (i.e. one copy of the variant is sufficient to manifest the trait) or recessive (i.e., two copies are required). For polygenic traits driven by many different variants scattered across the genome, however, whether the associated variants act in an additive or dominant fashion is less clear. To explore this question, Duncan Palmer, Alex Bloemendal, Ben Neale, and colleagues developed dominance LD score regression (d-LDSC) and applied it to UK Biobank data on 1,060 traits finding that for most complex traits dosage-dependent additive effects account for the overwhelming majority of heritable trait variation.