The centimorgan (cM) is a unit in genetic genealogy, measuring genetic distance rather than physical length. It helps understand the amount of shared DNA between individuals, indicating the closeness of their biological relationship. Genetic testing companies provide results in centimorgans, a standard metric for those exploring their family history. The total shared centimorgans with a DNA match indicate how closely two individuals are related.
Understanding the Centimorgan
A centimorgan (cM) is a unit of genetic linkage, quantifying the likelihood of genetic recombination between two points on a chromosome. One centimorgan signifies a 1% chance that two markers on a chromosome will separate during meiosis, the process where egg and sperm cells are formed. This separation occurs through crossing over, where segments of genetic material are exchanged between homologous chromosomes. The higher the centimorgan value between two genetic markers, the greater the chance of recombination.
Unlike physical measurements, a centimorgan does not represent a fixed physical distance in DNA base pairs. While one centimorgan roughly corresponds to about 1 million base pairs, this can vary across chromosome regions. The cM reflects the frequency of recombination, which is a more meaningful indicator of genetic relatedness than a simple base pair count. It provides a standardized way to measure how tightly linked genes are and how likely they are to be inherited together.
How cM Values Predict Relationships
The total number of shared centimorgans correlates with the closeness of a genetic relationship. A higher shared cM value indicates a more recent common ancestor and a closer familial bond. For instance, a parent and child typically share around 3,400 centimorgans, representing approximately half of their total DNA. Full siblings share 2,300 to 3,000 cM, though this can vary due to the random nature of DNA inheritance.
First cousins share between 700 and 1,300 cM, while second cousins share between 70 and 300 cM. These ranges are not exact but indicate potential relationships. For example, a shared cM value of 575 to 1,330 could suggest a first cousin, half-aunt, or grandparent relationship, necessitating further genealogical investigation to pinpoint the exact connection. DNA testing companies use these cM ranges to estimate potential relationships.
Factors Affecting cM Totals
The total shared centimorgan count for a relationship varies due to the randomness of genetic inheritance. During meiosis, the process that creates reproductive cells, DNA undergoes recombination, where segments are shuffled and exchanged between chromosomes. This random reshuffling means that even full siblings, who inherit DNA from the same parents, receive slightly different combinations and amounts of DNA, leading to variations in their shared cM. Consequently, two individuals with the same genealogical relationship may not share the exact same amount of DNA.
Another factor influencing cM totals, particularly for distant matches, involves “identical by descent” (IBD) versus “identical by state” (IBS). IBD refers to DNA segments inherited from a recent common ancestor. IBS segments are genetically identical but not inherited from a recent shared ancestor; their similarity is due to chance or ancient common ancestry. Small shared segments, especially below 7-10 cM, might be IBS rather than IBD, potentially inflating total cM values and suggesting a relationship that is more distant than indicated. DNA testing algorithms account for these small segments to provide accurate relationship predictions.
Applying cM to Genealogical Research
Centimorgan values are a tool in genealogical research, helping to confirm family trees and identify common ancestors. When a DNA match appears, the total shared cM provides a starting point for determining possible relationships. If a match shares 400 cM, consulting a shared cM chart can narrow down potential connections, such as a second cousin, a great-aunt/uncle, or a first cousin once removed. This allows researchers to focus their efforts on specific branches of their family tree.
Beyond total cM, examining the number and length of shared DNA segments offers additional insights. Longer shared segments typically indicate a closer relationship, as DNA segments tend to break down and shorten with each passing generation due to recombination. A match with several long segments is a stronger indicator of a close relationship than a match with the same total cM spread across many small segments. For very distant matches, shared cM values become less reliable, and the chance of random segment matches increases. Populations with endogamy, or intermarriage within a group, can show inflated shared cM values, as individuals may share DNA from multiple, more distant common ancestors, making relationship interpretation more complex.