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In genetically admixed populations, admixed individuals possess ancestry frommultiple source groups. Studies of human genetic admixture frequently estimateancestry components corresponding to fractions of individual genomes that traceto specific ancestral populations. However, the same numerical ancestryfraction can represent a wide array of admixture scenarios. Using a mechanisticmodel of admixture, we characterize admixture genealogically: how many distinctancestors from the source populations does the admixture represent? We considerAfrican Americans, for whom continent-level estimates produce a 75-85% valuefor African ancestry on average and 15-25% for European ancestry. Geneticstudies together with key features of African-American demographic historysuggest ranges for model parameters. Using the model, we infer that ifgenealogical lineages of a random African American born during 1960-1965 aretraced back until they reach members of source populations, the expected numberof genealogical lines terminating with African individuals is 314, and theexpected number terminating in Europeans is 51. Across discrete generations,the peak number of African genealogical ancestors occurs for birth cohorts fromthe early 1700s. The probability exceeds 50% that at least one Europeanancestor was born more recently than 1835. Our genealogical perspective cancontribute to further understanding the admixture processes that underlieadmixed populations. For African Americans, the results provide insight both onhow many of the ancestors of a typical African American might have beenforcibly displaced in the Transatlantic Slave Trade and on how many separateEuropean admixture events might exist in a typical African-American genealogy.

On the number of genealogical ancestors tracing to the source groups of an admixed population