ABSTRACT. A theory to account for across-founder ancestral relationships is presented, which can consider relationships within and across populations. The theory assumes finite size of the ancestral population. Ancestral relationships can be represented as metafounders, pseudo-individuals that can be seen as pool of gametes and as extensions of random genetic groups. Simple rules exist for computation of relationships, inbreeding, and the inverse of the relationship matrix. Values of the Ancestral relationships can be inferred from marker relationships and pedigrees. Once they are computed, use in Single Step GBLUP is straightforward and compatibility of marker and pedigree information is automatic.