Encountering the notation "cm" on an ancestry report can initially cause confusion, as it looks like a scientific measurement rather than a genealogical one. In the specific context of genetic ancestry, "cm" stands for "centiMorgan," a unit used to describe the length of DNA segments. This measurement is fundamental to understanding how ancestry composition and DNA matching work, as it quantifies the amount of shared genetic material between individuals. Unlike the metric system's centimeter that measures physical distance, the centiMorgan measures genetic linkage and the probability that a segment of DNA was inherited from a common ancestor.
Understanding the CentiMorgan Unit
The centiMorgan is named after geneticist Thomas Hunt Morgan and is based on the frequency of genetic recombination. In practical terms, one centiMorgan represents a one percent chance that a specific marker on a chromosome will undergo recombination during the formation of gametes. When you look at your ancestry DNA matches, the total centiMorgan count (often listed as "cM") indicates the total amount of DNA you share with a relative. A higher cM count generally signifies a closer biological relationship, as more DNA segments have been passed down from a common ancestor without being broken up by recombination events.
How cM Relates to Relationship Strength
Genetic genealogists have established general ranges of shared cM that correspond to specific relationship types. These ranges help users interpret the results provided by testing companies. For example, a match sharing between 800 and 1200 cM is typically a close relative such as a grandparent, aunt, or half-sibling. Matches sharing 200 to 350 cM often point to a first cousin relationship, while segments falling between 50 and 100 cM usually indicate a more distant connection, such as a second cousin. These values are statistical averages rather than strict rules, as the inheritance of DNA is a random process that varies from person to person.
cm in Chromosome Mapping
Beyond matching, "cm" plays a crucial role in chromosome mapping, a process used to identify which specific segments of DNA you inherited from which ancestor. Ancestry tools often feature graphical representations of chromosomes that display these segments as colored bars. The length of these bars is measured in centiMorgans, allowing you to visualize the exact pieces of DNA you share with a confirmed relative. By comparing these segments across multiple matches, you can triangulate your ancestry and determine whether a specific DNA segment originated from your maternal or paternal side, effectively building out your family tree one chromosome at a time.
The Difference Between cM and SNP Count
It is important to distinguish centiMorgans (cM) from SNP count when reviewing your ancestry data. While cM measures the physical length of shared DNA, SNP count (Single Nucleotide Polymorphisms) measures the total number of individual test locations that were compared. A match might show a high SNP count but a low cM if the shared DNA is spread across many small segments, or vice versa. Testing companies usually report both metrics, but the cM value is the more accurate indicator of the closeness of the relationship, as it accounts for the actual probability of inheritance rather than just the number of comparison points.
