DNA testing helps individuals understand their genetic heritage and connect with relatives. A common outcome is a percentage indicating shared DNA, which raises questions about the relationship. Interpreting these percentages is complex due to how genetic material is passed down.
How DNA is Passed Down
Genetic material is inherited from both parents, with offspring receiving approximately half of their DNA from each. This inheritance is not a simple halving of the entire genome; instead, it involves a process called genetic recombination. During recombination, chromosomes from each parent shuffle and exchange segments, creating a unique combination of genetic information in each child. This mechanism explains why siblings, despite sharing the same parents, do not inherit exactly the same DNA from them.
The amount of shared DNA is often measured in centimorgans (cM), a unit that represents the likelihood of recombination between two points on a chromosome. A higher centimorgan value indicates more shared genetic material. While a parent and child will share about 3400 cM, or roughly 50% of their DNA, the random nature of recombination means that the exact amount of DNA shared with more distant relatives can vary. This variability can make precise relationship determination based solely on percentages challenging.
DNA Percentages Across Generations
The expected amount of shared DNA decreases with each generation separating two individuals from their common ancestor. For example, full siblings typically share around 50% of their DNA, or between 2200 and 3020 cM. Grandparents and grandchildren, as well as aunts, uncles, nieces, and nephews, usually share about 25% of their DNA, or between 1340 and 2150 cM. First cousins, who share a set of grandparents, generally share an average of 12.5% of their DNA, which translates to approximately 866 cM, though this can range from 396 to 1397 cM.
These percentages are averages; the actual amount of shared DNA can differ due to genetic recombination. As relationships become more distant, variability increases, making it harder to pinpoint an exact relationship based on percentage alone. Beyond second cousins, distant relatives may share no detectable DNA segments.
What a 7% DNA Match Means
A 7% shared DNA match typically corresponds to a range of approximately 450 to 600 centimorgans (cM). This amount of shared DNA suggests a common ancestor who is a few generations removed. While a 7% match does not pinpoint a single specific relationship, it commonly indicates connections such as first cousin once removed, second cousin, or even half-first cousin.
For example, a first cousin once removed (the child of your first cousin) typically shares an average of 6.25% of DNA (102-980 cM). Second cousins, sharing great-grandparents, generally share about 3.13% of DNA (41-592 cM). A half-first cousin, sharing one grandparent, might share an average of 449 cM (156-979 cM). This overlap means a 7% match could represent several relationships, often requiring further investigation.
Confirming Your DNA Relationships
While shared DNA percentages provide valuable clues, confirming the exact nature of a relationship often requires additional steps. The total amount of shared centimorgans, especially the length and number of shared DNA segments, can offer more precision than percentage alone.
Building a family tree is an effective way to confirm DNA relationships. Mapping known ancestors and descendants helps identify potential common ancestors with your DNA matches. Contacting matches and comparing family information also provides insights to identify shared lineage. If significant DNA is shared across multiple segments, triangulation can be used. This involves identifying three or more individuals who share a specific DNA segment, strongly suggesting a common ancestor.