Number of ancestors and length of identity-by-descent tracks over time

The divergence between genetic ancestors (those who have contributed to an individual’s DNA) and genealogical ancestors (individuals recognized as ancestors in a societal context) evolves significantly over time. This study focuses on small, insular populations, where the number of genetic ancestors can swiftly encompass the entirety of genealogical ancestors. Additionally, in these populations, runs of homozygosity (ROHs) frequently occur due to random chance. This study used a forward-in-time simulation based on the Wright-Fisher model to analyze genealogical and genetic relatedness, haplotype block lengths, and runs of homozygosity (ROH) across generations. In small, insular populations, the number of genetic ancestors rapidly encompassed the total number of potential ancestors, and ROHs occurred frequently due to genetic drift. Our results showed that while genealogical ancestors followed an initial exponential growth phase before stabilizing, genetic ancestors grew more slowly. This was especially the case when the initial population size was large. These findings highlight the high frequency of ROHs in smaller populations, providing insights into the genetic structure of historically isolated groups.