When a direct-to-consumer DNA test returns a list of genetic matches, the primary metric used to gauge the relationship is the amount of shared DNA. This shared genetic material is the physical evidence of a common ancestor, forming the foundation of genetic genealogy. Testing companies report this shared DNA as a specific number of centimorgans (cM), which directly correlates to the closeness of the relationship. This measurement helps individuals understand their familial connections.
Understanding Centimorgans (cM)
A Centimorgan (cM) is the standard unit used to measure the length of shared DNA segments between two individuals. It is a statistical measure of genetic linkage, reflecting the likelihood that two points on a chromosome will be separated during recombination. Specifically, one centimorgan is equivalent to a one percent chance that two genetic markers will become separated in a single generation due to a crossover event.
The human genome contains an estimated 6,800 to 7,400 cM of DNA, meaning 24 cM is a very small fraction of the total. The greater the total number of shared centimorgans, the closer the individuals are related genealogically. Conversely, fewer cM shared indicates a more distant relationship. This unit provides a precise way to compare genetic matches, unlike simple percentages which can be misleading for distant connections.
The Specific Meaning of 24 cM Shared DNA
A match of 24 cM is considered a very small amount of shared DNA, placing the relationship in the distant cousin category. The range for a fourth cousin is typically 6 to 20 centimorgans, while fifth through eighth cousins can share as little as 6 cM. This level suggests the most recent common ancestor is likely four to eight generations back.
Relationships that may share approximately 24 cM include fifth cousins or fourth cousins once removed. These distant connections are typically categorized as being at the 8th-great-grandparent level or further back. It is often difficult to find the common ancestor in a typical family tree due to the sheer number of generations involved.
The probability of a close relationship being correct with only a 24 cM match is statistically low. First or second cousins would share cM amounts in the hundreds. Therefore, 24 cM points to a distant familial link that is highly unlikely to be closer than a fourth cousin.
Limitations of Small Shared Segments
The primary challenge in interpreting very small shared segments, like a 24 cM match, is the increased possibility of the DNA being Identical By Chance (IBC). IBC occurs when two individuals share an identical DNA sequence, but did not inherit it from a recent common ancestor. This random genetic similarity results in a “false positive” match in a genealogical context.
The reliability of a shared segment depends heavily on its length, not just the total cM amount. A single long segment of 24 cM is a more confident indicator of a true genealogical relationship than if the 24 cM were composed of several tiny segments, each less than 7 cM. Segments smaller than 7 cM have a high probability of being IBC and are generally not useful for tracing family lines.
Many testing companies filter out matches below a certain threshold, such as 7 or 8 cM, to reduce low-confidence results. When 24 cM is the total, the measurement is often composed of small segments, making the match less reliable for pinpointing a recent ancestor. The practical use of a 24 cM match is usually limited to confirming a distant connection found through traditional genealogical research.