Cryptic distant relatives are common in both isolated and cosmopolitan genetic samples.

Area of Science:

• Genetics
• Population Genetics
• Bioinformatics

Background:

• Close familial relationships can be inferred using a few hundred single nucleotide polymorphisms (SNPs).
• High-density genome-wide SNP data enable the inference of more distant relationships, such as 2nd to 9th cousinships.
• Understanding genetic similarity and kinship over 100-300 years is crucial for various genetic studies.

Purpose of the Study:

• To characterize the relationship between genetic similarity and the degree of kinship within a 100-300 year timeframe.
• To analyze the sharing of DNA inferred to be identical by descent (IBD) across diverse populations.
• To determine the implications of detecting distant relatives for genetic genealogy, forensics, and genotype/phenotype mapping.

Main Methods:

• Analysis of DNA inferred to be identical by descent (IBD) in large datasets (23andMe and HGDP-CEPH).
• Comparison of IBD sharing across 121 ethnolinguistically and continentally defined populations.
• Extensive pedigree-based simulations to establish bounds for predicted relationship degrees based on genomic IBD sharing.

Main Results:

• Average IBD sharing differs significantly between ethnolinguistic groups (e.g., Native Americans, Finns, Ashkenazi Jews) and continental populations.
• Pedigree simulations provided bounds for predicting relationship degrees from genomic IBD sharing in various population types.
• Tens of thousands of 2nd to 9th degree cousin pairs were identified within a sample of 5,000 Europeans.

Conclusions:

• Distant relatives are ubiquitously detectable via IBD segments in both ethnolinguistic and large 'unrelated' population samples.
• The findings have significant implications for the fields of genetic genealogy, forensics, and genotype/phenotype mapping studies.
• High-density SNP data provide a powerful tool for uncovering complex familial structures and population genetic patterns.