A DNA match is another tester whose results overlap with yours by an amount measured in centimorgans (cM). Every major testing company converts that number into a predicted relationship, shown as a single label or a range. The label is only a starting estimate, though, since several different relationships sometimes share nearly the same amount of DNA. Working out which ancestor actually connects you takes a few more steps beyond the first prediction you see.
The Predicted Relationship Can Fit More Than One Answer
Predicted relationships are calculated mainly from the total cM you share with a match. Some ranges are tight and distinctive: Ancestry reports that parent-child pairs almost always share between 3,100 and 3,500 cM, and full siblings almost always share between 2,200 and 3,000 cM. Other ranges overlap heavily. Ancestry's own research groups a grandparent, a grandchild, an aunt or uncle, a niece or nephew, and a half-sibling together. All of them typically share somewhere between 1,300 and 2,200 cM, with no way to separate them by total alone. A first cousin, a great-grandparent, a great-aunt or great-uncle, and a half-aunt or half-uncle cluster together in much the same way, usually sharing 650 to 1,200 cM. The same total, in other words, can point to several different places on your tree.
Shared Matches Narrow the Field
Every major company also shows which of your other matches also match a specific person, usually called shared matches or "in common with." Ancestry, 23andMe, FamilyTreeDNA, and MyHeritage each offer some version of this. Ancestry also weighs the pattern of DNA shared among a match's own shared matches, together with age and segment data. The goal was one higher-confidence relationship label instead of several possibilities listed at once. A shared match is a clue, not proof, however: two of your matches can appear on each other's shared-match lists while actually connecting to you through two completely different ancestors.
Family Trees Fill in the Connection
A linked family tree, when a match has one, is often the fastest way to spot a shared surname, place, or generation. Some companies also generate an automated theory of how you connect, built from other members' trees and historical records rather than from DNA itself. MyHeritage calls its version the Theory of Family Relativity; Ancestry calls its version ThruLines. Treat any such theory as a lead to check, not a proven pedigree, and verify each generation it proposes against an actual record before adding it to your own tree. Sometimes a match has no tree at all, which just means building one from that person's name, location, and any surnames they have listed.
A Shared Segment Still Needs a Documented Ancestor
More advanced researchers use a tool such as DNA Painter to map matching segments onto a chromosome browser. This shows which relatives share DNA at the same location, a technique called triangulation. A shared segment narrows a relationship further, but it does not by itself name the common ancestor. Two people who both match you on the same segment are not necessarily related to each other on that segment's line. Endogamy and pedigree collapse can also inflate shared totals well past what a typical chart predicts, sometimes making a distant cousin look like a much closer one. The relationship counts as confirmed only once the DNA evidence and a documented paper trail agree on the same shared ancestor.
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