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Pre-Columbian Mixtec social organization was distinguished by the tight endogamy of the ruling class, which included many consanguineous marriages. It was also characterized by a vigorous historical and genealogical tradition. The historical documents, or codices, provide materials for the calculation of the levels of inbreeding present before the Spanish Conquest. A genealogical analysis of inbreeding was performed on the combined pedigree, which spanned the tenth through sixteenth centuries, of all individuals connected by ancestry, descent, or marriage with Lord 8 Deer Jaguar Claw of Tilantongo (a .d . 1063-1115). Sixty of the 217 couples (27.65%) were consanguineous. When only couples of wholly known grandparentage were considered (N = 39), F = 0.1051. The mean F of all couples, even those where one spouse was of unknown parentage, was 0.0243. Over the 550 years of the pedigree the maximum F in any 52-year period was 0.1324. This level of inbreeding is sufficient to produce noticeable effects on population structure and affinities over time.