7
Nov

Total Centimorgans Shared vs. Shared DNA Segments

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If you have been examining your DNA match list, you might have noticed that there are a few different details related to the amount of shared DNA that you have with any particular match.  What is the difference between total centimorgans and the number of DNA segments that you and your match share?  Which bit of information is most important to figuring out how you and your match are connected?  Does sharing more segments mean that you are more closely related?

In this post, I’ll answer those questions – and more!

Note:  Ancestry DNA customers might notice that they can only access the total number of shared segments and total amount of shared DNA, but won’t be able to see details about segment size and location of segments.  If you are interested in viewing this type of information, you might consider uploading your raw DNA information (which can be downloaded from Ancestry DNA) to another site that has a chromosome browser.

What is the difference between total centimorgans shared and shared DNA segments?

When DNA testing companies analyze our DNA, they compare positions in our genome to positions to those of their other customers in order to identify possible genetic matches.  When two people show up as a match, the testing company is able to measure a genetic distance between two points where the DNA is identical, and this is reported as a matching DNA segment. 

It is possible to share only one DNA segment with someone, or more than a few dozen individual segments.

The sum of the genetic length of the DNA segments that you share with a match is reported as the total number of centimorgans (abbreviated as cMs) that you share.  Companies usually only include DNA segments that are larger than a specific threshold in the calculation for total shared centimorgans. 

This is because the smaller the DNA segment, the higher the probability is that it is identical-by-state (coincidentally identical) vs. identical-by-state (identical because it was inherited from a common ancestor). 

Each company has a different threshold for how big a DNA should be to be included in the sum of total centimorgans shared, but it is typically somewhere between 5-7 cMs.

The size and location of each DNA segment, as well as the total amount of shared DNA, can help you figure out how you and your DNA match are related.  Additionally, if you and a known relative have both taken DNA tests, you can use the total centimorgans shared data to determine whether or not you share the expected amount of DNA for your relationship type.

Does sharing more segments mean two people are more closely related?

The number of DNA segments that we share with someone, considered with the size of said segments, can offer us clues to to how distantly we might be related, or whether or not we are likely to be related in more than one way.  With that said, there is no rule about how many segments you should share with a particular relative, and you can’t come to any definitive conclusion about how you are related to someone based simply on the number of shared segments.

For example, if you share many very small (under 10 cM) segments with a match, it is possible – yet not guaranteed – that you might be related to that person distantly in more than one way.  This occurs frequently in communities that are descended from endogamous populations, such as Colonial Americans, Native Americans, French Canadians, or the European Jewish.

Conversely, if you share only one long 50 cM segment with a match, there is a possibility that you are as closely related as second cousins, but also as distantly related as fourth cousins.

Unfortunately, sharing many DNA segments with someone doesn’t always mean that we are more closely related.  For example, two full siblings will usually share fewer (and longer) DNA segments than half-siblings, yet are more closely related.

It would be amazing if there were some sort of key that we could create that would tell us how many DNA segments two people of a particular relationship distance should share, but unfortunately such a tool will probably never exist..

Does sharing more total centimorgans mean that two people are more closely related?

The total number of centimorgans that you share with a match usually is a good indicator of how closely you are related to someone.  Even so, it’s important to note that you can share many, many small DNA segments equaling a fairly high amount of total centimorgans.

Take, for example, the following image:

What does endogamy look like in Mexican DNA

The person who took the Family Tree DNA test and Julie share a total of 201 centimorgans, but the longest segment is only 7 cMs in length.  If I only looked at the 201 cMs figure, I might erroneously believe that Julie is a relatively close cousin of the person who gave the DNA sample. 

Even Family Tree DNA suggests that they might be 2nd-3rd cousins!  But, because the longest segment is only 7 cMs, I know that it is most likely that they are distant cousins possibly sharing several common ancestors on different lines of their families.

Other times, I can be sure that the total amount of shared centimorgans is leading me in the right direction about the possible relationship between two people who have taken a DNA test. The following image is a screenshot from the Gedmatch Genesis One-to-One comparison between two people who discovered that they are half-sisters:

DNA shared between half sisters

As you can see, they share a large number of total centimorgans (1631 cMs).  Not only that, the size of the largest segment is 97.4 cMs, which is very long.  It is almost impossible to share a DNA segment this long with someone and not have a fairly recent ancestor (in this case, a parent).

Total shared centimorgans and the size of the largest DNA segment is the most helpful information to determine your relationship to a match

To summarize, I would like to leave you with the following points:

  • Neither the amount of total centimorgans shared or the number of shared DNA segments is enough information to help you figure out how you are related to someone – or how you are not related to them.
  • While the total amount of shared centimorgans is more useful than the number of shared DNA segments, it is possible to share many small segments with a distant match, which is why the size of the longest segment is important.
  • In order to estimate your relationship to someone, you will need to take into consideration your family tree, their family tree, your total amount of shared DNA, and the size of the longest DNA segment shared.

Conclusion

I hope that this post has given you a better understanding about the difference between the total centimorgans shared, the number of shared DNA segments, and how to use this information to see how you might be related to your DNA match. 

If you have any questions about something that you read in this post, or would like to ask a specific question about a DNA match that you have, I look forward to hearing from you in the discussion below.

Thanks for stopping by today!

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Total Centimorgans Shared vs. Shared DNA Segments
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Total Centimorgans Shared vs. Shared DNA Segments
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What is the difference between total centimorgans and the number of DNA segments that you and your match share? Which bit of information is most important to figuring out how you and your match are connected? Does sharing more segments mean that you are more closely related?
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Who Are You Made Of
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