Ancestry Centimorgans and Segments (and Longest Segments)

First, Ancestry gave us centimorgans and segments. Now, they’re showing unweighted shared DNA and the longest segment. What does it all mean, and how can we use this information in our research? This article is an illustrated guide to these genetic genealogy concepts in clear and understandable terms.

How to Find Centimorgans and Segments on Ancestry

The most basic explanation of centimorgans and segments is that they describe how much DNA you share with your genetic matches. Before we dive into terminology, let’s look at how Ancestry displays this information.

The DNA match list is ordered by shared DNA. You can see the number of centimorgans and segments at a glance. Here’s my fourth Cousin Jack, with whom I share 20 cM on 1 segment:

The same information is displayed when you open the match profile.

But things get a little more interesting when you realize that the numbers in both places are a clickable link. Judging by comments on genealogy forums, this isn’t obvious to many experienced Ancestry members.

When you click the numbers, Ancestry has long displayed a list of possible relationships that this amount of shared DNA may represent. But let’s focus on some recent additions to this pop-up display.

How to Find the Longest Segment on Ancestry

You find the longest segment on Ancestry clicking on the numbers link shown in the previous image.

Underneath the shared DNA, we now see “Unweighted shared DNA” and “Longest segment”.

I’m showing a simpler example where 1 segment is involved, and these two extra measurements are the same. That’s not always the case. But what’s also interesting here is that these numbers of 29 cM are larger than the shared DNA of 20 cM.

How can that happen? Before we get into that, I’m going to explain exactly what centimorgans and segments represent. Use the table of contents to skip these sections if you are familiar with these terms.

What are Centimorgans and Segments?

Let’s step away from the Ancestry display for a moment, and deal with the underlying genetic science.

Cousin Jack and I have inherited the same piece of DNA from a common ancestor. The more DNA we share, the closer our relationship. We measure amounts of shared DNA in centimorgans.

You have about 6,800 total centimorgans of DNA, with about half coming from each parent. So, you share about 3,400 cMs (centimorgans) with each parent and similar amounts with full siblings. The numbers are never exact, due to the random nature of inheritance.

My fourth Cousin Jack and I share a rather small amount of DNA, which we can map out on a specific chromosome. Our shared DNA is nestled on chromosome #12, which is one of our 23 pairs of chromosomes. I’ll break it all down using this schematic diagram:

The diagram chops up the chromosome into minute positions and shows (in purple) where our shared DNA starts and ends.

A Simple Explanation of Segments

The start and end position of shared DNA defines a single segment.

So, there you have the meaning of a segment. I only share one segment with Jack.

In an alternative scenario, we could also have shared DNA elsewhere on the chromosome. Or we might have a shared piece on a completely different chromosome. The start and end positions of shared DNA marks out those additional segments.

A Simple Explanation of Centimorgans

Segments will vary in length, and we want to know how long these segments are. The diagram chops up the shown segment into 20 genetic units. Those are the 20 shared centimorgans between myself and Cousin Jack. Simple as that!

Centimorgans aren’t really a measure of distance or length. You’ll get along fine if you think about them as a consistent unit of shared DNA, where Cousin Jack at 20 cM is a more distant relation than Brother John at 3,460 cM. If you want to get more technical, check out the definition from the International Society of Genetic Genealogy.

A Video Guide to the Ancestry Longest Segment

Here is a companion video to this article. The rest of this article covers the topic in more depth.

What is the Ancestry Longest Segment?

The longest segment is quite a simple concept, but the display on Ancestry may be a little unclear at first. Let’s look at the simplest explanation first. We’ll come back to Cousin Jack later, but we’ll switch focus now to my more distant Cousin Virginia.

This is a real example. I’ve chosen Virginia because she has also tested with MyHeritage. Unlike Ancestry, MyHeritage has a chromosome browser which shows me which chromosomes have our shared DNA. That lets me give you a more accurate diagram.

Here is Cousin Virginia on Ancestry:

We share 19 centimorgans across 2 segments, with the longest segment being 11 cM.

The MyHeritage numbers are very similar (this is not always the case). More specifically, MyHeritage tells me that our shared segments are on Chromosome #1 and #17. The larger segment is on #17.

Definition of the Longest Segment

The longest segment is simply the shared segment with the highest number of centimorgans. Ancestry displays the longest segment as 11 cM. Virginia and I only share two segments, so I can infer the approximate size of the other segment.

What about the other segments?

The Ancestry display doesn’t let me infer other segment sizes when there are more than two involved. Take my closest relative on Ancestry:

Ancestry tells me that our longest shared segment is 45 cM, but I have no idea how the rest of our shared DNA is spread across the other 19 segments. That’s because Ancestry does not provide a Chromosome Browser. There are ways around this – see my article on four ways to use a chromosome browser with your Ancestry DNA matches.

You may not have forgotten the elephant in the room, my Cousin Jack (sorry, Jack). We share a total of 20 cM across 1 segment, but the longest segment is reported as 29 cM. What the heck? Before we get to that, we need to look at that other figure in the Ancestry display: “Unweighted shared DNA”.

Timber and Unweighted Shared DNA

There are several reasons you may share a piece of DNA with a genetic match in the Ancestry database.

What you want for genealogy research is that the DNA is identical by descent (IBD). This means that you inherited the DNA from a common ancestor.

There is also a possibility that the DNA is identical by chance (IBC). The smaller the segment, the more likely this will occur. This is what leads to false positives in your low cM matches i.e. people with who you share DNA with but who are not genealogically related to you at all.

But we won’t worry about that thorny issue in this article. There’s a third type of DNA sharing we need to address.

Identical by Population

That third type is IBP, or identical by population. This refers to a segment of DNA that is common across a particular ethnicity or population group. Yes, there is a common ancestor. Indeed, there may be many common ancestors in different lines. But these ancestors are so far back in time, that you will not be able to evaluate or identify them using DNA matches.

For a detailed explanation, go check out this article from the DNA Explained blog.

Ancestry’s Solution: Timber!!

Ancestry’s DNA processing attempts to filter out shared DNA that is identical by population. Their algorithm is called Timber.

Let’s take another look at Cousin Jack and his strange numbers.

Ancestry DNA processing found 29 cM of shared DNA between us. Then Timber goes to work.

The Timber algorithm trawls through the Ancestry database to see who else shares those centimorgans on that particular chromosome pair. The algorithm has decided that a fairly sizeable 9 cM is prevalent across a particular group of people and is due to some very distant ancestor.

A quick look at the ethnicity tab tells me that Jack and I only share the Ireland region of Ancestry’s ethnicity groupings.

I assume that Timber is slicing out a segment of DNA that is prevalent amongst many other DNA testers with Irish heritage.

Definition of Unweighted Shared DNA

The unweighted shared DNA is therefore the number of shared centimorgans before the Timber algorithm is applied.

How Can the Longest Segment Be Bigger than the Total Shared DNA?

It’s also clear that the Longest Segment is showing the unweighted number. This is why it can be bigger than the total shared DNA.

I’m not sure that Ancestry will continue with this display. It’s just confusing. They should either change the labels or show more numbers i.e. all the unweighted and weighted numbers.

How Useful is the Longest Segment?

Now that you know what it is, the question becomes: do you need to know? In other words, is this information useful when researching your DNA matches?

Not necessarily. If you do not have significant levels of endogamy in your ancestry, then you can gloss over this measure. Keep focusing on the shared centimorgans in the main display.

However, it becomes far more useful when your research must account for endogamy across your lines.

Using the Longest Segment with Endogamy

Endogamy refers to significant levels of inter-marriage within a group for many generations. This may be due to reasons such as an isolated location, religious traditions or cultural norms. On a practical level, it may become evident to you if you identify multiple marriages across different branches of your family tree.

Even if you can’t decipher prior generations, it may become apparent if you share “too much” DNA with known relatives. This will happen if you are related to, for example, a second cousin through multiple branches on both your paternal and maternal lines.

Some groups and communities have a growing resource of research and articles to help genealogy research with endogamous heritage. You’ll find many excellent articles that focus on Jewish or Acadian genealogy.

I’ll also give a shout-out to this detailed article on Polynesian heritage. There are lots of screenshots from Ancestry, as the researcher uses the new longest segment size to analyze DNA matches.

The guideline is to use the longest segment to zone in on DNA matches that are worth researching for a recent common ancestor. The longer the segment, the more likely that you share a more recent ancestor with the DNA match. Well, that’s a little vague. How long is long enough?

There’s no hard and fast rule, but a figure of 20 cM is often mentioned by genealogists who research endogamous trees.

Centimorgan Charts

When you click on any centimorgans link, the pop-up window gives you a breakdown of the possible relationships at that number of centimorgans.

The display is laid out in descending order of probability. Here’s the top two for myself and Jack.

Judging by the questions on genealogy forums, Ancestry testers don’t find this display very helpful. I agree. I can’t easily visualize where a “half 2nd cousin 2x removed” would fit into my tree.  So, this is not my first port-of-call when I’m trying to figure out a relationship.

My Favorite Online Centimorgan Chart – Shared cM Project

I like charts that arrange centimorgans in the style of a family tree. My go-to online chart is the Shared cM Project on the DNA Painter website. The chart is interactive (roll your pointer over it). And when you enter a cM total into the input box, it highlights the possible relationships in the tree presentation.

My Favorite Offline Centimorgan Chart – the Green Chart

If you want a chart to use offline or to print out, then you can’t go wrong with the “green” version from the DNA Detectives Facebook group. Here’s a link to a copy outside of Facebook. The DNA Geek blogger has permission to host the image.

Differences Between Charts

Different charts use different estimates for relationships. These differences become apparent in the upper and lower ranges of the estimates of how many centimorgans may represent a relationship.

Blaine Bettinger’s Shared CM project (mentioned earlier) is compiled from user-submitted data and is more likely to cover “unusual” edge cases when compared to the chart from DNA Detectives. And Ancestry’s predictions are partly proprietary.

Just be aware that the edges between ranges will overlap, and these charts provide a frame for your genealogical research.

Centimorgans And Percentages

Sooner or later, you’ll realize that shared DNA can also be reported in terms of percentage. For example, 23andMe displays percentages for your DNA matches in their database. If you’re used to one format, it can be difficult switching mentally to the other.

The close relationships are clear enough. You share about 50% of your DNA with a sibling and about 25% with a grandparent.

Personally, I much prefer to use centimorgans, because I have very few close DNA matches on either Ancestry or 23andMe. When you’re down in the weeds, the percentages drop below 1%. I find these more difficult to distinguish between one and the next.

How to Convert Centimorgans to Percentages

It’s easy to do an approximate conversion between the two representations. I mentioned that we all have about 6,800 centimorgans in our entire DNA.

To convert any figure of centimorgans into a percentage, divide the cM by 6,800 * 100. Or just divide by 68 for less typing into that calculator.

You share 3,450 cM with Brother John? That’s 3450/6800*100 = 50.7%.

Cousin Jack and I share 20 cM, which is 20/68 = 0.29%.

How to Convert Percentages to Centimorgans

MyHeritage, bless them, show both percentage and centimorgans in their match display. But 23andMe show percentages in their shared match display.

To convert the percentage to the approximate number of centimorgans, just multiply by 68.

Quick Approximate “How Many” Answers

These are averages without the ranges. Use them for a quick check, but be sure to evaluate the upper and lower levels given by the more complex charts in the prior section.

Query%cMs
How many centimorgans shared with a parent?50%3,400
How many centimorgans shared by siblings?50%1,700
How many centimorgans shared between half-siblings?25%1,700
How many centimorgans shared with an aunt or uncle?25%1,700
How many centimorgans shared with grandparents?25%1,700
How many centimorgans shared with a first cousin?12.5%850
How many centimorgans shared with a first cousin once removed?6.25%425
How many centimorgans shared with a second cousin?3.13%213
How many centimorgans shared with a 2nd cousin once removed?1.5%102
How many centimorgans shared with a third cousin?0.78%53
How many centimorgans shared with a fourth cousin?0.2%14

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Margaret O'Brien

1 thought on “Ancestry Centimorgans and Segments (and Longest Segments)”

  1. Thank you! But what about when longest segments is higher cM’s than total or maybe fills up total close to top and ancestry has identified several eg more than one and I’ve seen up to four segments and am left totally confused wondering where on earth Ancestry has fitted in the other 2 or 3 segments with no, or only like 3 cM of space left in total outside of longest identified?? And where total number of segments hasn’t changed, but now can see unweighted which I’ve seen up to twice as big in cM as total? And in some I’ve seen no change in total or number of segments and only difference is can now see length of largest (total and weighted exactly the same) and appears room for other segments to fit? Will appreciate if you can explain what each of these scenario’s mean.

    Reply

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