Due to genetic linkage, segments of unrecombined DNA pass down through the generations intact. These segments are identical by descent (IBD) down to at least 2 cM in size and have an unknown common ancestor approximately 25 generations ago. By starting with a known ancestor and detecting the common IBD segments through triangulation, multiple IBD segments can be identified and combined into an autosomal genetic signature.
Through the evaluation of allele frequencies, the probability of a random combination of allele matches can be calculated. Under ethnically homogenous conditions, one to one autosomal DNA matches can result in a false positive. Under those same conditions, triangulation demonstrates that a false positive is statistically improbable, indicating the match is identical by descent (IBD).
The process of genetic inheritance is often over simplified, leading consumers of genetic tests to believe that the amount of DNA from distant ancestors becomes negligible. In fact, segments of DNA pass down through the generations intact. Naturally occurring cleavage sites allow for small segments to exist at recurring chromosomal locations. These small segments can be used as familial markers in an autosomal haplotype.