Shared Centimorgans

What is a centimorgan and how many do we share with our ancestors?
Marc McDermott

In this article, I want to discuss what exactly centimorgans are. As they are the general measurement for shared DNA, I hope this will be as helpful for beginners as for more advanced researchers.

I will go over the following:

  • What is a centimorgan (cM)?
  • How many cM’s do we share with certain family members?
  • How to determine your relationship to matches using your shared amount of cMs
  • What is a segment and why do they get smaller?

What is a centimorgan?

The easiest way to answer this question is with another question. What is an inch? A form of measurement right? Well, this is the case with centimorgans (cM’s). Now, since I went over the simple explanation, I am going to get into the really deep explanation.

Centimorgans are the amount of shared or identical SNPs (single nucleotide polymorphisms) along a segment. We have chromosomes and on those chromosomes have shared segments. We will inherit different segments from different parents and grandparents so, when comparing segments to relatives, we will have shared segments composed of thousands of SNPs. When we have identical segments, they are measured in centimorgans. Centimorgans and SNPs are measured in tandem within thresholds. This can be very confusing so let us use an example.

Measuring centimorgans

Let’s use a measuring tape for instance. You have measurements laid out on a measuring tape (or ruler) and imagine those lines as cM’s – they measure distance. SNPs, think of them as fixed locations which represent each individual atom that makes up the measuring tape. Thousands of atoms lay between each marking on the measuring tape. When we have a DNA match that shares the exact same atoms along their measuring tape, we measure those in cMs. We will have more identical atoms on our measuring tape with closer relatives than with distant relatives thus, the sections of the measuring tape are larger.

The above photo is an example I created to help us visualize this better. Tommy Wilkins shares a 13.6cM on his second chromosome with his DNA match Matt Wilkins. You will see that the 13.6cMs are measured across 1,688 SNPs. We call this section of their shared chromosome of Segment. Larger segments mean closer relationships to our matches.

Shared centimorgans

For most basic use of cM’s it is important to pay attention to the number of shared cM’s and the number of shared segments. All DNA companies have different thresholds for which they measure our matches. These thresholds are below:

CompanyMinimum cMsMinimum SNPs
GedMatch7cM default but can be changed by user to a minimum of 3cM700 default – can be adjusted
FamilyTreeDNA7cM for any single segment + 20cM total500

These thresholds help weed out minute matches. If we had a low 1cM thresholds, we would likely seem to be more closely related to someone than we actually are. We, as humans, are all closely related in the long run so putting these thresholds in place help us maintain proficient accuracy.

How many centimorgans do we have?

Each individual has about 6,800 cM in total. Of these 6,800 they inherit about 3,400 or more from each parent. So, depicting how much you share with your family member, this graph below is quite helpful. There are a plethora of useful charts online that you can use by a simple google search.

cM RangecM AverageRelationship
36003,600Parent / Child
2300 – 39002650Full Sibling
1300 – 23001800Half Sibling Aunt/Uncle/Niece/Nephew  Grandparent/Grandchild
575 – 1400900First Cousin  (1C) ½ Aunt/Uncle/Niece/Nephew Great Aunt/Uncle/Niece/Nephew Great Grandparent / Great Grandchild
215 – 650450First Cousins Once Removed (1C1R) ½ 1C ½ Great Aunt/Uncle/Niece/Nephew  
75 – 3602242C 1C2R ½ 1C1R
30 – 2151122C1R ½ 2C 1C3R ½ 1C2R
0 – 109563C 2C2R
0 – 75303C1R or great distant cousin

I highly recommend using DNAPainter’s Shared cM project by Blaine Bettinger. It gives you the most likely scenario in how you relate to your DNA matches. AncestryDNA and 23andMe algorithms do not account for half relations or generational removals. Using this tool will better help you determine your relationship with each individual. It is your due diligence to determine your relationship. Where Ancestry might display your relationship with a DNA match to be 4th cousins, you very well could be much closer as 2C2R.

The Shared cM Project offers an interactive cM chart that allows you to easily visualize your relationships with your genetic cousins. This can be accessed here at DNApainter.

Why do genetic segments get smaller?

As our autosomal DNA is passed from parent to child, our parent’s genetic profiles combine together in creating our own unique genetic make-up. We call this recombination. As each segment gets passed down, they generally break up and become smaller.

In the segment photo at the top of the article where Tommy Wilkins and Matt Wilkins display their second chromosome segment of 13.6cM, it is likely that their father’s shared, with each other, a larger segment. In a perfect world, people assume they inherit half of their DNA from each parent. If this was truly the case, we would assume that Tommy and Matt’s father’s segment was about 27.2cM. However, DNA is imperfect and this is not always the case. Just an example of how recombination affects the size of our segments.

Think of segments like a stone that gets passed down from generation to generation. Imagine that each generation the stone is passed down to that a bit of it gets chipped away and it becomes smaller. Let us say that two brothers are passed a stone from their father that is very similar in size. When they pass this stone on to their children, it too will become smaller but they do not get chipped in the exact same areas.

Recombination is entirely random. When segments go through recombination they can even break up in two. This is a very common occurrence.

One of the biggest things to remember is that smaller segments will not appear if the amount of cMs are below the cM threshold. This helps keep at bay those DNA matches that might contain ancient DNA segments that we call “sticky segments”. These segments are so small that they often forego recombination and hang around for generations.

A general rule is that you cannot tell how you relate to someone using just the amount of shared segments, however it can be very useful. Always remember that the cMs shared between you and a DNA are the best way to determine your relationship.

Final thoughts

With the above-discussed topics regarding cMs, SNPs, charts, and segments, I hope you will be able to further your family’s history alongside furthering the understanding of your genetic networks to ultimately increase your genealogical findings. Just remember to pay attention to the amount of cM’s and the number of segments you share with DNA matches and cross-reference those numbers in DNA Painter’s Shared cM Project. This will help you prove your exact relationship to these cousins using the most accurate algorithm that the major DNA sites do not have. Happy findings!

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