What does cM Shared Mean in DNA Results?

If you're looking at DNA results from Ancestry or 23andMe, you'll probably be wondering what cM shared means. In this article, we'll help you to understand this genealogy measurement and how it is used for tracing family matches.

You’re looking at your family matches from AncestryDNA or 23andMe and you are seeing numbers with a cM next to them. What is this abbreviation? What does it mean? The simple answer is that it is a unit of measurement by which we can determine the length of certain DNA segments.

This cM unit is very important when it comes to our familial matches regarding our genetic connections. Understanding shared cM can be the key to cracking a family history mystery and I have a couple of instances in my own family where I have been able to burst through a brick wall using this type of matching.

What Does cM Mean?

There can often be a little confusion when you first see the cM unit as obviously it is very similar to the length unit cm or centimeters. Although they are both measurement units, cM is vastly smaller than centimeters. The term cM refers to the unit of measurement centimorgans.

The centimorgan is named after the American born geneticist Thomas Hunt Morgan. It is a unit of measurement used to measure genetic linkage. One centimorgan is equivalent to a 1% chance that a DNA marker on a chromosome can become separated from another marker on that same chromosome. This is measured over the course of one generation of genetic inheritance.

In terms of DNA length, a centimorgan is equivalent to around a million base pairs as found in our DNA sequences. So what does shared cM mean?

Shared cM Explained

The reason you came looking for this answer is likely because you were looking at your matches on AncestryDNA or some other major site and saw the term “Shared cM.” Without a little context, it is hard to understand what this means so let me briefly explain.

We now know what a centimorgan (cM) is so what does it mean to have a shared cM? Firstly, we must remember that the information these testing sites are supplying to us is still just an estimate even though it does reflect our DNA as well as the DNA of others.

A match we see on our testing site shows an estimate of the shared cM range we have with one of our matches. So to clarify what this means we need to consider the cM as a physical measurement of the length of a segment of DNA.

If we have a segment of DNA that is, say 23 cM long, and our match has that exact same segment, then this is a centimorgan match. We may share multiple identical segments of DNA with a match which we would add to get our total centimorgan match.

We may have long and short segments in common with our matches and they may be found in very different places across our entire genome. We inherit these segments of DNA from shared ancestors, for example, we may share great-great grandparents and be third cousins.

When we share an identical segment of DNA usually over 6 centimorgans long then we are usually assessed to be potentially related to a match. Anything below 6 centimorgans could be a very distant match or merely coincidental.

Why Size Is Important in Shared cM Matches

A million base pairs are equal to about one centimorgan but each person has about 6,800 centimorgans of DNA length. As with all things cellular, these centimorgans are a tiny measurement but are still highly significant.

The closest familial match is that of parent-child because we receive half of our DNA from each of our parents. As a result, we share around 3400 centimorgans of DNA with both of our parents. However, these are not just two long segments of 3600 cMs.

When we inherit DNA from our parents segments of DNA come from each and randomly recombine within our own DNA. We may share long segments with our parents but we also may have multiple short and long segments as well.

Large shared cM matches overall indicate a close familial relationship. There is in fact a chart created by Blaine Bettinger which gives us a rough estimate of the amount in centimorgans we might share with different family members.

A first cousin who would be the child of one of our aunts or uncles for instance can share between 553 – 1225 cMs of DNA in common with us. Our own siblings share between 2209 – 3720 cMs. Identical twins of course would be on that higher end.

This is why then it is important to know how many centimorgans we share with a specific match if we want to determine how we are related. High levels mean they are closely related and lower amounts mean there may be a distant connection.

How to Find Shared DNA on AncestryDNA

Locating where your shared centimorgan values are found on AncestryDNA is pretty easy. As I mentioned you likely saw them before coming in search of what exactly they meant. For those who have not yet found them here is a brief description of where they are found.

To find your shared cM values start at the Ancestry home screen and choose the DNA section in the top toolbar. In the dropdown menu choose “Your DNA Results Summary.” This takes you to a page with your three main DNA tools.

When you choose the middle tool which should say "View all DNA Matches" you will be taken to a list of all your potential relatives. Against each name, you will see your estimated centimorgan match and percentage DNA match.

These two numbers can be very beneficial in estimating how you are related to your match and potentially how closely. The higher the numbers the more closely you are related to those individuals. All you need to do now is hope they have a family tree on the site and you might be able to confirm or debunk the estimates you are given.

Where to Find Shared cM on 23andMe

When using 23andMe it can be a little trickier to see your shared centimorgans. This is due to users having the option to hide certain information. If privacy settings allow you will need to visit the 23andMe chromosome browser tool to find this information.

This chromosome browser is known as the Advanced DNA comparison tool and will be found by hovering over the “Friends and Family” tab at the top of the navigation menu.

Where to Find Shared cM on Family Tree DNA

Just like AncestryDNA with Family Tree DNA, you do not have to search too hard to find your shared cMs. They are listed alongside the names of all your matches They are found in the column which unsurprisingly is called “Shared cM.”

This site is also like 23andMe in that it also has its own version of a chromosome browser. This will allow you to find out more information regarding how your shared cMs breakdown. This would include how many identical segments you share along with other helpful information.

Can You Use Shared cMs to Determine Your Relationship to a Match?

As mentioned, the number of shared centimorgans you share with a person can help you estimate your possible relationship with them. The closer you are related the higher your shared cM count will be.

It is generally easy to spot a sibling or parent match, this is because the numbers are so high. We do have to understand though that as you get further away from your immediate family these estimates become less reliable in guessing the match.

I have a third cousin on AncestryDNA which based on our shared cM number more closely matches a fourth cousin range. We both have accurate trees and have been able to determine who our shared ancestors are.

This also works the other way because I have a second cousin who I can trace through the records accurately but they fall closer to a first cousin match based on our shared cM range. It isn’t exact and it does require some supporting old school genealogy research to confirm just how you are related.

Conclusion

Shared centimorgans (cM) are a measurement of your estimated relationship to your DNA matches. They refer to segments of DNA found in your genomes that match up with those of your matches. The higher the cM match the more closely you are likely to be related.

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