The question of how long genetic relatedness persists across generations is answered by the mechanics of DNA inheritance and the limitations of modern testing. Relatedness is defined by the amount of shared genetic material inherited from a common ancestor. This shared DNA diminishes with each generation, eventually reaching a point where it is no longer detectable by standard consumer genetic tests. The speed of this genetic dilution determines the practical limit to tracing family connections through DNA alone.
The Genetics of Shared Ancestry
The foundation of genetic relatedness lies in Mendelian inheritance, which dictates the predictable halving of shared DNA with each subsequent generation. Every person inherits approximately half of their DNA from each parent, which equates to an average of about 3,700 centimorgans (cM) of shared DNA with each biological parent. This consistent reduction means that the expected percentage of shared DNA drops rapidly as the distance to a common ancestor increases.
The centimorgan (cM) is the unit of genetic measurement used to quantify shared DNA between two individuals. A higher cM value indicates a closer relationship, representing longer, intact segments of DNA passed down from a shared ancestor. For instance, the expected share drops from 50% with a parent to about 25% with a grandparent and roughly 12.5% with a great-grandparent.
This exponential decrease in shared genetic material is due to recombination, where chromosomes shuffle and exchange segments during the formation of reproductive cells. Each generation, the length of the shared DNA segments gets progressively smaller as they are broken up and randomly mixed with unshared segments. This mechanism ensures that the percentage of the genome inherited from any single ancestor decreases rapidly.
The Threshold for Undetectable Relatedness
The point at which shared DNA becomes undetectable by commercial testing provides a practical answer to the question of lost relatedness. While mathematically the shared DNA never truly reaches zero, it becomes statistically insignificant when the remaining segments are too small to be reliably identified. Most consumer genetic testing companies use a minimum threshold for shared DNA to identify a match, which is typically set between 7 and 10 cM.
Shared segments below this threshold are often discarded because they are likely “identical by chance” rather than “identical by descent” from a recent common ancestor. These tiny segments can be misleading, representing ancient shared ancestry rather than a genealogically useful connection. Consequently, a genetic relationship is considered lost when the expected shared DNA falls below this minimum segment size and total cM.
This loss of detectable shared DNA generally occurs around the sixth to tenth cousin level, depending on the specific line of descent. By the time two individuals are estimated to be eighth cousins, the probability of sharing any detectable DNA segment is low. The randomness of DNA inheritance means that many distant relatives who are genealogically connected will not appear as a match on a DNA test.
Distinguishing Genealogical Ties From Genetic Evidence
It is important to differentiate between a genealogical tie, which exists on a family tree, and a genetic relationship, proven by shared DNA. Genealogy relies on historical records to trace every ancestor who contributed to one’s lineage. Genetic relatedness requires the physical inheritance of measurable DNA segments from a common ancestor.
Because DNA inheritance is random, it is possible for two people proven to be seventh cousins through records to share zero common DNA segments. After about seven generations, a person has received DNA from only a portion of their genealogical ancestors. This means an ancestor may be part of the family history but not a genetic ancestor who contributed DNA to a living descendant.
The genealogical family tree includes every person who contributed to one’s existence, while the genetic family tree only includes ancestors from whom one inherited DNA. Therefore, a lack of a genetic match for a distant cousin does not disprove the documented genealogical relationship. The DNA test simply fails to detect the genetic evidence of that distant connection.
The Concept of Universal Ancestry
If one traces ancestry back far enough, relatedness shifts from individual genetic segments to universal genealogical connection. The theoretical number of ancestors doubles with every generation, leading to an impossibly large number of ancestors after only a few centuries. For example, going back 30 generations suggests over a billion direct ancestors, far exceeding the historical world population at that time.
This mathematical impossibility is corrected by “Pedigree Collapse,” which occurs when ancestors appear in multiple places in one’s family tree. This happens when relatives intermarry, such as when cousins reproduce, reducing the total number of unique ancestors. Pedigree collapse stops the exponential growth of unique ancestors, meaning the halving of shared DNA is not infinitely sustainable in a closed population.
Due to pedigree collapse, the genealogical lines of all humans eventually converge. The concept of the Most Recent Common Ancestor (MRCA) suggests that all living people share at least one common ancestor who lived in the relatively recent past. While genetic segments from this MRCA would have been lost long ago, every person is ultimately related to every other person through a vast, overlapping web of genealogical connections.