24
Jun

All About DNA Segments

Have you checked your DNA matches lately? If you have, you may have seen that you share DNA “across segments” with your matches.

In this post, find out what it means to share DNA across different numbers of DNA segments. Plus, learn what each DNA segment represents, as well as whether the number of shared DNA segments is important.

You’ll even find out whether the size of DNA segments matters.

There is a lot to learn when it comes to understand your DNA match list and how your genetic relatives are related to you. Fortunately, the concepts are fairly simple.

My goal is to help you understand enough of the basics to be able to spot where your DNA matches are located in your family tree.

What are matching DNA segments?

Matching DNA segments are identical pieces of DNA shared between two people. These identical DNA segments are located on specific locations on an individual chromosome.

The size, or length, of matching DNA segments is usually measured in “centimorgans“, or cMs. Identical DNA segments can be very small or very large.

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What does “across segments” mean in DNA matches?

Included before the term “across segments” is the total number of shared DNA measured in centimorgans. Therefore, “across ‘x number of’ segments” means the total number of centimorgans across the total number of DNA segments shared between two individuals.

For example, if you see a DNA match that shares 38 cMs across 3 segments with you, you know two things:

  • The total amount of DNA that you share with this person is 38 centimorgans
  • You share a total of three DNA segments

In order to know the size of each DNA segment, we would need to use a chromosome browser to compare our DNA to our DNA matches.

Some DNA testing companies, such as 23andMe, My Heritage, and Family Tree DNA, include a chromosome browser as a part of your DNA results.

If you did not test with these companies, you and your match can always upload your DNA to Gedmatch for free in order to compare your DNA with the free chromosome

What does each DNA segment represent?

Each DNA segment that we share with our matches was likely inherited from a shared common ancestor. We will usually share multiple DNA segments with closer relatives, which were all passed down from our recent common ancestors.

We may only share one or two segments with a distant cousin. Our more distant common ancestor may have passed down one or both of these segments to both us and our cousin.

It is possible to be related to a DNA match in more than one way. Sometimes, this means that we share multiple DNA segments with a match because one or two DNA segments were passed down from our multiple common ancestors.

How many DNA segments should you share with a match?

There is no specific number of DNA segments that we expect relatives to share. Generally speaking, very close relatives could share dozens of DNA segments and distant cousins may share only one or two.

For example, I share 28 DNA segments with my full first cousin, and only 6 DNA segments with a second cousin once-removed.

If you have a half-relationship with a cousin, you might notice that you share fewer DNA segments with them than you do with your full cousins of that same relationship.

For example, I share only one small segment with a half-second cousin once-removed. Compare this to the 6 segments shared with the full second cousin once-removed that I mentioned above.

Is the number of shared DNA segments important?

The number of DNA segments doesn’t really tell us much, however. For example, you could share dozens of very small DNA segments with DNA matches that are not closely related to you.

Instead, we prefer to focus on the size of the shared DNA segments in order to determine how far back in our family tree we should look to find our common ancestor.

Does the size of DNA segments shared between matches matter?

While we don’t usually pay much attention to the total number of shared DNA segments shared with a match, the size of shared DNA segments can provide important clues as to how distant our most recent common ancestors might be.

There is no exact rule about size of DNA segments and how many generations back in our tree we need to look to find our common ancestor. Instead, we only know that smaller DNA segments usually mean a distant common ancestor and larger segments mean a more recent common ancestor.

What constitutes a large or small DNA segment?

While there is no exact definition, the below general guidelines are what I use to help me sort DNA segments among my matches:

  • Less than 12 cMs = small
  • Between 12-50 cMs = medium
  • Between than 50-100 cMs = large
  • Longer than 100 cMs = very large

If we share large or very large segments with our matches, we can be sure that we only need to look a few generations back in our tree to spot our common ancestor.

Those DNA matches that share very small to medium-sized segments are more difficult to research. The smaller DNA segments are, the higher the chance that they will be passed down completely intact to the next generation.

This means that the smaller the segment, the higher the possibility that the common ancestor is located many generations back in the family tree. Sometimes, it can be so far that there are no genealogical records to help us research the connection.

In fact, there is a 6% chance that a 7 cM segment can be 1,000 years old. This is why I usually recommend focusing on DNA matches that share segments larger than about 12 centimorgans for research.

How many DNA segments are there in humans?

Each person has approximately 6800 centimorgans in their 22 numbered chromosomes. There is no specific number of DNA segments that a human has, however, because only portions of these chromosomes will be identically shared with genetic relatives.

Conclusion

I hope that this post has helped you understand more about DNA segments shared across your chromosomes, and how you can use this information to understand how you are related to your DNA matches.

If you have any questions about something that you read in this post, or if you would like to share a specific question about shared segments with a match, please join us in the discussion below.

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