The phasing process began with the daughter’s DNA profile. A child inherits fifty percent of their DNA from each parent — specific segments, passed down through the process of meiosis in which chromosomes are combined and recombined across generations. The segments the daughter inherited from her mother, Lori, are present in her genome alongside the segments she inherited from her father, Blake Ruff.
Because Blake Ruff’s DNA profile was known — he had submitted a sample as part of the investigation — it was possible to identify which segments of the daughter’s DNA had been inherited from Blake and which had been inherited from Lori. The segments not inherited from Blake must have come from Lori. This reconstruction is an approximation — phasing is a probabilistic method, and the reconstructed maternal profile is not identical to what Lori’s own DNA profile would show — but it is sufficiently accurate for genealogical database searching.
When the reconstructed maternal profile was searched against the 23andMe and Ancestry databases, it produced matches with other individuals who shared DNA segments. These matches represented relatives — people who shared a common ancestor with Lori Ruff, and who had voluntarily uploaded their genetic data in search of ancestry information.
The matches ranged in relationship distance. A first or second cousin would share a substantial proportion of DNA, enough to be identified with high confidence. A third or fourth cousin shares much less, and the genealogical research required to connect the match to the unknown subject becomes correspondingly more extensive.
The early appearance of the Cassidy name in the matches was significant, because the Cassidy name pointed toward a specific family rather than a common surname. Fitzpatrick’s work over the subsequent months and years involved building family trees from the matches — identifying common ancestors, tracing those lines forward through generations, and looking for candidates who matched the approximate age and geographic background of the unknown subject.