The extent to which parent of origin (the inheritance of genetic variation from the mother or father) contributes to the heritability of complex traits is unknown. Now, Richard Mott and colleagues use data from a mouse heterogeneous stock population to show that the majority of 97 complex traits have measurable parent-of-origin effects (Cell 156, 332–342, 2014). Heterogeneous stock mice were bred from 8 inbred progenitor strains and maintained for over 50 generations. The authors used available phenotype and genotype data to measure parent-of-origin effects on trait heritability, finding such effects for 91 of 97 (93%) traits. The authors analyzed 837 known quantitative trait loci (QTLs) for the 97 traits and found that QTLs with parent-of-origin effects included imprinted loci and loci not known to be imprinted. They further investigated two QTLs not known to be imprinted by selecting a candidate gene within each QTL and performing reciprocal crosses of knockout alleles. These analyses confirmed that Man1a2 has parent-of-origin effects on body weight and that H2-ab1 has parent-of-origin effects on a percentage of CD4+ T cells. The authors suggest that parent-of-origin effects may be generated by altering in trans the expression of imprinted genes.