The question of whether two people can share DNA without being genealogically related often arises from at-home ancestry test results. The answer depends entirely on how “sharing DNA” and “related” are defined. While all people are genetically connected in the context of deep human history, recent family history requires distinguishing between different types of genetic sharing. Understanding the technical definitions used in genetics is necessary to reconcile a DNA match notification with the absence of a known common ancestor.
The Universal Truth of Shared DNA
All human beings share a profound genetic connection, which is the baseline truth of our species. This deep, ancient relationship means that any two people selected at random are approximately 99.9% genetically identical. The vast majority of the human genome is dedicated to fundamental biological functions necessary for life, which are conserved across all individuals.
This massive amount of shared DNA is a direct result of our common evolutionary origin, establishing universal relatedness. The difference between two people is confined to the remaining fraction of a percent, which accounts for all the variation in human traits, such as eye color or disease susceptibility. While this shared bulk of the genome means every human is related, this genetic fact is too broad to be useful for family history.
Defining Genetic Relatedness in Ancestry Testing
Commercial DNA testing companies define a meaningful relationship by looking for signs of recent common ancestry, a much stricter standard than evolutionary sharing. This genealogical relatedness is measured by identifying shared segments of DNA, quantified using the centimorgan (cM) unit. A centimorgan is not a physical distance but a measure of the statistical likelihood that two DNA segments will be separated during inheritance.
When two people share a significant number of centimorgans, it suggests they inherited that identical segment from a recent common ancestor. For example, first cousins typically share around 1,000 cM. Even distant cousins sharing an eighth great-grandparent may still share small amounts of DNA. The amount of shared cM is what the testing company uses to estimate the closeness of the relationship, such as identifying a second or third cousin.
The Critical Distinction: Identity by State vs. Identity by Descent
Sharing DNA without being recently related relies on the distinction between Identity by State (IBS) and Identity by Descent (IBD). Identity by Descent (IBD) is the standard for genealogical relatedness. It means the shared DNA segment was passed down directly from a recent, known common ancestor. This segment is identical because it has not been split or altered by recombination since being inherited from that shared ancestor.
Conversely, Identity by State (IBS) describes DNA segments that are identical in sequence but are not inherited from a recent common ancestor. The two people happen to have the exact same DNA sequence at the same location, either due to random chance or inheritance from a very distant ancestor. While all IBD segments are also IBS, an IBS segment is only considered IBD if it originates from a recent ancestor.
The existence of IBS is why a DNA test can report a “match” even when a recent family connection is absent. This identity by chance is more likely to occur with shorter DNA segments. The probability of a random, identical sequence matching another person’s increases as the segment length decreases. Testing company algorithms must interpret whether a shared segment is a true IBD match or merely a coincidental IBS match.
Factors That Create False Sharing
Several genetic and demographic factors increase the likelihood of two people sharing DNA segments that are Identical by State (IBS) rather than Identical by Descent (IBD). The size of the shared segment is the most important indicator. Segments below a certain centimorgan threshold are highly likely to be false positives. Although the exact threshold varies, segments under 7 to 10 cM are frequently considered too small to reliably indicate a recent common ancestor.
Another significant factor is endogamy, which occurs when a group marries within a small, isolated community over many generations. Examples include specific religious or geographically isolated groups. This repeated intermarriage dramatically inflates the amount of shared DNA within the group, as everyone is distantly related through multiple ancient lines.
The total amount of shared DNA in an endogamous population can appear much higher than the actual genealogical relationship suggests. A distant cousin may look genetically like a much closer relative. In these cases, the multiple small segments are IBS, originating from ancestors far beyond the typical genealogical cutoff. When evaluating a match from an endogamous population, genetic genealogists often focus only on the largest shared segment, requiring it to be over 20 cM to confirm a recent relationship.