Shared DNA (autosomal DNA) is the measurable portion of genetic material that matches another person, indicating a shared common ancestor. Consumer genetic tests often provide a percentage reflecting this shared material. The amount of DNA you share indicates how closely you are related in your family tree. This article explores the mathematical principle governing genetic inheritance to determine the generational distance corresponding to a 10% shared DNA reading.
The Law of Halving Inheritance
The foundational principle of genetic inheritance dictates that, on average, the amount of DNA shared with an ancestor decreases by half with each successive generation. You receive approximately 50% of your DNA from each parent, establishing the baseline for this progression.
Moving back one generation further, you inherit about 25% of your DNA from each of your four grandparents. Continuing this pattern, you would expect to share an average of 12.5% of your DNA with each of your eight great-grandparents. This predictable halving creates a statistical model for tracing ancestral distance.
For a second great-grandparent, which is four generations back, the average shared percentage is 6.25%. This consistent reduction is crucial for genetic genealogists and for the algorithms used by DNA testing companies to estimate relationships.
Calculating the 10% Threshold
To determine how many generations back a 10% shared DNA result falls, we compare it to the established averages. The average for a great-grandparent is 12.5%, while the average for a second great-grandparent is 6.25%. Since 10% falls between these two figures, the common ancestor is likely a great-grandparent, or a relationship branching off at that generational level.
A 10% shared DNA result typically corresponds to a “close family” relationship. It is commonly found in first cousin relationships, who share a common set of grandparents (two generations back), or with a great-aunt or great-uncle.
Because genetic inheritance is not perfectly precise, a 10% match falls within the expected range for a first cousin (8% to 22%) or a half-aunt/uncle. This relatively high amount of shared DNA indicates a common ancestor who is only two or three generations removed, typically involving the great-grandparent or great-great-grandparent generation.
Why Actual Shared DNA Varies
While the law of halving provides a useful average, the actual amount of shared DNA between relatives can deviate significantly due to the random nature of genetic recombination. Recombination shuffles genetic material when reproductive cells are formed, meaning the 50% you inherit from a parent is a randomized mix of their parents’ DNA. Consequently, you may inherit slightly more than 25% from one grandparent and slightly less from the other.
This randomness means the 10% figure is a statistical expectation, not a guaranteed amount. For this reason, genetic testing companies rely on a more precise unit of measurement called the centimorgan (cM). A centimorgan is a unit of genetic linkage that estimates the probability of recombination occurring between two points on a chromosome.
The total amount of shared centimorgans provides a reliable metric, with the percentage being a more general, user-friendly conversion of that total. Beyond the total shared DNA, the length and number of “identical by descent” (IBD) segments are also analyzed. Longer, fewer shared segments generally indicate a more recent common ancestor, while many small segments suggest the shared DNA is from a more distant past. Segment analysis helps refine the estimated relationship when the total shared percentage falls into an overlapping range.