At-home DNA testing kits have made it easier to explore personal ancestry and connect with distant relatives. Understanding what the percentage of shared DNA actually signifies about a relationship is often challenging. When a test reveals approximately 7% shared DNA, this figure points toward a specific range of familial connections, but it does not provide a definitive answer. Translating this percentage into a precise generational distance requires an understanding of how genetic material is passed down and the natural variations that occur. The expected amount of shared DNA decreases with each generation, creating a mathematical framework for tracing lineage.
The Mathematics of DNA Inheritance
The foundation of genetic genealogy rests on the predictable halving of shared DNA with each passing generation. Every person inherits exactly 50% of their DNA from each biological parent. This predictable reduction forms the theoretical baseline for calculating shared DNA with more distant ancestors.
Moving back one generation, a person is expected to share 25% of their DNA with each grandparent. This trend continues, with the theoretical average halving again to 12.5% for a great-grandparent. A great-great-grandparent would, in theory, contribute 6.25% of a person’s DNA.
A genealogical generation is typically defined by the parent-child step. The expected shared percentage for any ancestor can be calculated by dividing 50% by two for every step away from the parent. This calculation provides the theoretical expectation for determining generational distance. For instance, an ancestor six generations back, such as a fourth great-grandparent, is expected to share approximately 1.56% of your DNA.
Translating 7% Shared DNA into Generations
A shared DNA percentage of 7% falls between the theoretical 12.5% expected for a great-grandparent and the 6.25% expected for a great-great-grandparent. This places the common ancestor, based purely on the average, somewhere in the range of three to four generations removed from you. A 7% match most frequently corresponds to a relationship that shares a common set of great-great-grandparents.
In terms of specific relationships, 7% shared DNA often indicates a first cousin once removed, a half first cousin, or a second cousin. The theoretical average for a first cousin once removed is around 6.9%, which aligns closely with the 7% figure. This means the common ancestor is likely a grandparent or great-grandparent of one individual and a great-great-grandparent of the other.
The 7% shared DNA suggests a common ancestor who is four generations back for one person and five generations back for the other. A relationship where the common ancestor is a third great-grandparent is expected to yield about 3.125% shared DNA, and a fourth great-grandparent about 1.56%. The 7% figure is a relatively strong connection.
Genetic Variability and the Range of Possible Relationships
While the theoretical averages are useful, the actual amount of shared DNA can fluctuate significantly due to genetic recombination. Recombination is the random shuffling of DNA segments that occurs when reproductive cells are formed. This random shuffling means that a person does not receive a perfect, even quarter of DNA from each grandparent, but rather a unique combination that can skew the final percentage.
Genetic testing companies rely on a more precise unit of measurement called the centimorgan (cM), with percentage being a simple conversion of the centimorgan total. Centimorgans measure the length of shared DNA segments, providing a more accurate scale for relationship prediction. A 7% shared DNA result typically equates to a range of approximately 450 to 600 centimorgans, depending on the testing company’s calculation model.
The ranges for different relationships frequently overlap, meaning 7% could potentially represent a few different connections. For example, a shared amount in this range could point to a first cousin once removed, a half first cousin, or even a second cousin once removed. This variability is more pronounced in distant relationships because the effects of recombination accumulate over generations.