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When you get an autosomal DNA test whether from AncestryDNA, 23andMe or Family Tree DNA, you can use the results to find possible DNA matches.
All genealogical DNA testing services use centiMorgans to estimate (emphasis on ‘estimate’) how close you are with someone else. Depending on the number of centiMorgans across your shared DNA segments they could be close family members such as a parent or grandparent or a relative further away such as a second cousin.
So what exactly is a centiMorgan (abbreviated as cM) and how do you use it to find and analyze DNA matches.
A centiMorgan is the measure of the probability of a crossover event. That is, the likeliness of two different DNA segments crossing over between two different DNA molecules.
Genetic recombination is how we get physical traits passed down from our parents.
Interesting fact: The term ‘centiMorgan’ was coined by Alfred Henry Sturtevant in honor of his teacher, Thomas Hunt Morgan, a famous American geneticist. The original unit was called a Morgan but is rarely used today.
A centiMorgan describes the frequency of recombination or crossover. Specifically, a centiMorgan represents a one out of a hundred chance (1%) that a crossover within a DNA segment will occur within one generation.
On average one cM is equal to around 1 million base pairs (the two nucleotides that make up one rung of the DNA helix ladder). That means that a crossover is likely to happen for every DNA segment consisting of 1 million base pairs.
The definition of a centiMorgan can be a bit confusing and many people tend to see it as a measurement of distance. It’s not. Rather, it is a measurement of probability.
It’s important to understand that a centiMorgan is not a definitive unit of measurement. This is because not all DNA segments have the same recombination frequency. Like a lot of things in genealogy, it’s a close estimate.
But it is very handy in determining the closeness of two people who share DNA. In fact, it may provide a more accurate prediction than just looking at shared DNA segments.
Most DNA testing services will include the number of number of centiMorgans in matching DNA segments. You don’t have to carry out any complicated calculations.
You’ll need to order a DNA test first. See our buying guide for an in-depth comparison of the top three DNA ancestry testing services.
After the results are in go to your results page in your secure account and check whether there are any matches.
In AncestryDNA for instance, you click on a specific match and then click or hover on the small i next to the confidence label. This brings up a box showing the number of centiMorgans on shared segments.
The higher the number of centiMorgans the more significant the match is. It means you both have inherited a number of traits from a recent ancestor.
There is a minimum number of cMs required to establish any kind of familial relationship. It’s usually 7cMs.
The higher the number goes the closer you are to your match. It can go as high as 3,000cMs for a very close match such as a parent or a child.
Here is a table by AncestryDNA showing how the number of shared centiMorgans corresponds with different levels of relationships.
As you can see, centiMorgans are not precise measurements. For instance most people share around 120cMs with a third cousins but in other third-cousin matches they can be as few as 90 or as high as 200.
Another way to express how much DNA you share is in form of percentages.
You can convert centiMorgans into percentages simply by dividing the total number of cMs by 68.
There are roughly 6,800 possible centiMorgans in a person’s DNA. So if you share 3,400 centiMorgans with a match you can say you share 50% of DNA (that is, 3400/6800*100 or simply 3400/68).
A 50% match is likely a parent or a child while a 1% match is likely a third cousin.
You shouldn’t use centiMorgans on their own to determine familial relationships. They can be misleading at times especially for more distant matches.
Combine what you learn from your 23andMe, FTDNA or AncestryDNA test with your family history and historical records to draw a more accurate timeline and establish more accurate ancestral links.
I'm just another amateur genealogist investigating my American-Scots-Irish lineage. I built MyFamilyDNATest.com after buying all of the leading DNA tests to discover everything I could about my family history. Hopefully, this site will save you time and demystify the emerging science of DNA-based genealogy, for your family project.