DNA testing services, like AncestryDNA, offer insights into genetic relationships, often presenting a metric known as “cM.” Understanding this unit is an initial step toward interpreting your DNA results and uncovering family connections, as it plays a central role in estimating kinship.
The Centimorgan Explained
A centimorgan (cM) is a unit of genetic distance, not a physical measurement like an inch or centimeter. It quantifies the likelihood of recombination, or crossover events, occurring between two points on a chromosome during meiosis. Specifically, one centimorgan signifies a 1% chance that two markers on a chromosome will become separated by recombination in a single generation.
This concept is important for understanding how DNA is inherited and shuffled from one generation to the next. In DNA testing, centimorgans refer to the length of shared DNA segments between two individuals. These shared segments indicate a common ancestor. A higher centimorgan value for shared segments generally suggests a more recent common ancestor and a closer genetic relationship.
Centimorgans and Your DNA Matches
AncestryDNA utilizes the total amount of shared centimorgans between two individuals to predict their relationship. For instance, a parent and child typically share approximately 3300 to 3720 cM. Full siblings generally share between 2200 and 3300 cM.
As relationships become more distant, the amount of shared DNA decreases, and the ranges of shared centimorgans broaden, leading to more overlap between different relationship types. First cousins typically share between 396 and 1397 cM, with an average of 866 cM. Second cousins might share an average of 221 cM, with a range of 55 to 490 cM. Beyond the total cM count, the length of the longest shared segment also contributes to Ancestry’s relationship prediction algorithm, as longer segments often suggest a more recent shared ancestor.
Factors Affecting Shared Centimorgan Values
Shared centimorgan values are not always perfectly consistent for a given relationship due to the random nature of DNA inheritance. Each child inherits a unique combination of DNA from their parents, meaning even full siblings will not share identical amounts of DNA. This inherent randomness contributes to variations in shared cM values among relatives of the same degree. Half-relationships, such as half-siblings, share less DNA than their full counterparts, which is reflected in their lower cM counts.
Certain population dynamics also influence shared cM values. Endogamy, where individuals within a group intermarry for generations, can lead to higher-than-expected shared centimorgans. This occurs because individuals are related through multiple ancestral lines, resulting in more, often smaller, shared DNA segments. Similarly, pedigree collapse, which happens when an individual’s ancestors are related to each other (e.g., through cousin marriages), reduces unique ancestors and can elevate shared cM totals, making a distant relative appear genetically closer.
Beyond the Centimorgan Count
While the centimorgan count provides a foundational understanding of genetic relatedness, other tools and considerations are important for comprehensive genealogical research. The “shared matches” feature, available on platforms like AncestryDNA, allows you to see other DNA matches that you and a specific match have in common. This can help identify common ancestral lines and narrow down potential relationships, especially for unknown matches.
Comparing family trees with your DNA matches is another powerful method to confirm or refine predicted relationships. If a match has a public or linked family tree, examining shared surnames or geographical locations can reveal the common ancestor. Ethnicity estimates offer a geographical context for your genetic origins, indicating regions where your ancestors may have lived. However, these estimates are probabilistic and can change as testing companies update their reference panels and algorithms. Combining centimorgan data with genealogical research provides a more complete picture of your family history.