Consumer DNA testing is a popular method for exploring ancestry and connecting with genetic relatives. A key measurement used to quantify shared DNA between you and a match is the centimorgan (cM). This unit translates the amount of shared genetic material into a probability that indicates how closely two individuals may be related. Understanding the centimorgan is essential for interpreting the relationship predictions provided by DNA testing companies.
Defining the Centimorgan
The centimorgan (cM) is a unit that measures genetic distance, not physical distance. It is a probabilistic measure derived from the frequency of genetic recombination, a natural process occurring during the formation of egg and sperm cells. One centimorgan represents a 1% chance that a chromosome segment will undergo a crossover event (recombination) and be separated from another point during a single generation. A higher cM value signifies a greater genetic distance because recombination events are more likely between points that are further apart.
The total amount of DNA in the human genome is estimated to be approximately 6,800 to 7,400 cM. This total is the basis for calculating shared DNA between relatives. Since you inherit half your DNA from each parent, you share about 3,700 cM with each biological parent. The centimorgan value is a statistical tool used to map the relative positions of genes and predict the likelihood of certain DNA segments being passed down together.
Translating cM Counts into Relationships
The total number of shared centimorgans is the primary factor used to estimate your genealogical relationship. A higher total cM count indicates a more recent common ancestor and a closer relationship. Relationships with a guaranteed direct link, such as parent/child, share the highest amount, typically between 3,330 and 3,720 cM, with little variation.
The amount of shared DNA becomes more variable with each step of genetic distance due to the randomness of inheritance. Full siblings share an average of about 2,629 cM, but the range is broad (2,209 to 3,384 cM). This wide range explains why two full siblings can have different amounts of shared DNA with other relatives. Half-siblings, aunts/uncles, and grandparents/grandchildren share an average of around 1,750 cM, with ranges that overlap significantly.
For first cousins, the average shared DNA is about 874 cM, with an expected range from 553 to 1,225 cM. This overlap means a match of 1,315 cM could potentially be a close first cousin, half-sibling, aunt, or grandparent, depending on the family structure. As the relationship becomes more distant, the cM range broadens, making it less precise to determine an exact relationship based on the total number alone.
Why Segment Length Matters
The total shared centimorgan count is the sum of all individual matching DNA segments. The characteristics of these segments provide additional context. Longer, continuous shared segments are stronger evidence of a recent common ancestor than the same total cM amount spread across many small segments. This occurs because each generation introduces more opportunities for recombination, which breaks up long segments into smaller pieces.
For close relatives, such as first cousins, you typically share fewer, but longer, segments. As the common ancestor becomes more distant, the shared segments become shorter and more numerous, reflecting the cumulative effect of recombination events. A segment length of 10 cM or larger is a strong indicator of a true genealogical connection, known as Identical By Descent (IBD), meaning the segment was inherited from a shared ancestor.
Segments shorter than about 7 cM are more likely to be Identical By State (IBS). This means the DNA sequences are identical by chance, not due to a recent common ancestor. For example, a match of 90 cM spread over a single 90 cM segment suggests a closer relationship than the same 90 cM spread over ten 9 cM segments. Most DNA testing companies filter out very small segments to minimize false matches and ensure the reported total cM reliably measures recent shared ancestry.