Abstract

Overall, in a diploid organism, the realized number of genetic ancestors of a focal individual is a tiny proportion of its potential ancestors. The number of genetic ancestors is bounded by the number of ancestral genome segments, which increases linearly with the length of the genome and with the number of generations depth. However, this bound is a small part of the overall picture of a genome's ancestry. First, surviving ancestral lineages are clustered in the tree of potential ancestral lineages, with subtrees of surviving lineages rooted at 8 to 10 generations for a typical mammalian genome. In consequence, in a geographically structured population, genetic ancestors are clustered geographically, and are far more local to the focal individual than the totality of ancestors. In addition, surviving lineages are not only clustered across a chromosome, but disjoint linked segments of a single lineage survive for many generations into the distant past. This results in the number of genetic ancestors of an individual remaining distinctly below the bound given by the number oof ancestral genome segments. These results have implications for the limited diversity of genomes from large historic populations that are now represented by a very small number of remnant individuals who may constitute the future of a species.