Centimorgans (cM)
Definition
The unit of genetic map distance used to measure shared DNA between individuals. A first-degree relative shares approximately 2,550 cM; a third cousin shares approximately 200-850 cM. The amount of shared DNA in centimorgans estimates the probable degree of relationship.
- Unit type
- Genetic map distance
- First-degree relative sharing
- Approximately 3,400 cM
- Third cousin sharing
- Approximately 45-215 cM
- Used in
- Investigative genetic genealogy
- Function
- Estimates probable degree of relationship
Common questions
Why is shared centimorgans only an estimate of relationship, not a certainty?+
Because recombination is random, the amount of DNA two relatives actually share can vary within a range for any given relationship type, so overlapping cM ranges between different relationship categories require additional genealogical research to resolve.
How does shared cM differ from a percentage of shared DNA?+
Percentage shared DNA is a simple fraction of the genome in common, while centimorgans measure genetic map distance across specific chromosome segments, which is the unit genealogy databases actually use to rank and categorize likely relationships.
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