For the purpose of identifying expedition members, this characteristic is valuable because it allows comparison across many generations. Nigel Gambier is separated from James Fitzjames by many generations in the paternal line — described as a second cousin five times removed — and yet their Y-chromosome profiles are expected to be very similar or identical, because the Y chromosome was passed down the male line from Sir James Gambier (the common ancestor) through multiple generations to both men without the recombination that affects other chromosomes.
The match between the NgLj-2 molar’s Y-chromosome profile and Nigel Gambier’s Y-chromosome profile is thus strong evidence that the two share a paternal-line ancestor — and because Tetteroo’s genealogical research had established that Fitzjames was the specific individual in that line whose remains would be expected at an Arctic site on King William Island, the researchers concluded the match established identity.
It is worth being precise about what “established identity” means here. DNA matching at the Y-chromosome level cannot, in isolation, definitively exclude all other possible explanations. In theory, another person who also descended paternally from the Gambier line and who also died on King William Island in 1848 could produce the same match. In practice, no other such person is known to have existed: the expedition’s crew roster is documented, the Gambier family connections of its members are known, and no other expedition member with Fitzjames’s specific paternal line has been identified. The researchers presented the DNA match in the context of this historical evidence, and the combination — documented remains at a known expedition site, specific Y-chromosome match with a living descendant of the person expected to have died there — was judged sufficient to establish identification.