Leeds Method: Why Do I Have More Than 4 Clusters? Or, Time to Consolidate!

The Question

Today I want to discuss one of the most frequent questions I get: Why did I get more than 4 color clusters on my Leeds Method chart? 

(I’ll address the issue of getting less than 4 clusters in a future post.)

The Simple Answer

There are basically two answers to this question.

1. 1. You don’t have enough second cousins. (We will also discuss this in a future post.)
   2. You need to do another step which is consolidating (or combining) some of your clusters.

Key Concepts

As we discuss this, let’s keep a few key concepts in mind:

1. 1. The goal of the Leeds Method is to naturally sort our DNA into clusters of matches who are biologically related to each other.
   2. We hope to get 4 clusters representing our 4 grandparent lines. However, not everyone will get 4 clusters! (In other words, your results may vary!)
   3. Many people need to consolidate (or combine) clusters to “see” their grandparent lines.

The Leeds Method

Let’s start with a simple, fictional Leeds Method chart which created 5 clusters using these basic steps:

1. 1. List all of your DNA matches between 400 and 90 cM.
   2. Assign the color blue to your highest match, Lori, and add an asterisk to her.
   3. Assign the blue to all of Lori’s Shared Matches: Joe, Rebecca, and Liam. (Note: A Shared Match is a person who shares DNA with both you and your match, Lori.)
   4. Assign the color red to the highest match that doesn’t already have a color, Meredith, and add an asterisk to her.
   5. Assign red to all of Meredith’s Shared Matches: Joe and Rebecca.
   6. Continue steps 4 and 5 until everyone has at least one color. (Note: These steps created the Green (Lynne), Yellow (Lucy), and Purple (Mitzi) Clusters.)

Overlap and Heavy Overlap

We’ve now completed our initial Leeds Method chart. We were hoping to see 4 clusters, although we know some people will have more or less than 4 clusters. Our chart has 5 color clusters.

But our chart shows overlap. In fact, it shows heavy overlap. So let’s define these two terms:

• • overlap –  a person with more than one color. In this example, Joe and Rebecca were assigned both the color blue and red, so they have overlap.
  • heavy overlap – most of the people in one cluster are also in another cluster. In this example, 2 out of 3 of the Red people are also in the Blue Cluster. Also, 2 out of 4 of the Blue people are also in the Red Cluster.

Whenever we see heavy overlap, as in this case, my recommendation is to combine those colors into one. In this case, we would combine the Blue and Red Clusters. This would give us a total of 4 clusters.

Next, I’ll explain why I recommend consolidating clusters. This is a more advanced explanation. If you want to keep things simple, you might skip this part! But if you want to understand why these clusters probably formed then continue reading.

Seeing the Segments

As you probably know, we cannot see the segments (or pieces) of DNA we share with our matches at Ancestry. This is because Ancestry does not have a chromosome browser. But let’s pretend we can see the segments of DNA using a fictional chromosome browser. For simplicity, we will only look at the 5 people who make up the Blue and Red Clusters. (Remember, those are the two clusters that have heavy overlap between them!) And, we will pretend that all of the matches showed up on one of your chromosomes: chromosome #1.

In this pretend scenario, let’s also assume that you have identified these 5 matches and know they are all related to you through your Grandpa Fred’s part of the family.

Below are the fictional chromosome browsers for you and your 5 matches we determined were biologically related to Grandpa Fred. Each chromosome is broken into 6 segments which are numbered from 1 to 6. The grey segments are those inherited from Grandpa Fred’s part of the tree.

The Blue Cluster

Now let’s think about how we created our chart. When we used the Leeds Method, Lori, who inherited segments #2-5 from Grandpa Fred, started our Blue Cluster. Her Shared Match list included the following people: Joe, Rebecca, and Liam. They showed up on her Shared Match list because they share DNA with both you and Lori. 

With this fictional chromosome browser, we can see that Lori shares the following DNA segments with these matches:

• • • Joe at segment #3 & #4
    • Rebecca at segment #3
    • Liam at segment #2 & #3

Note that, although Meredith also has DNA from Grandpa Fred’s part of the family (segments #1 & #6), Meredith and Lori don’t share any DNA segments. So, Meredith would not show up as a Shared Match of Lori’s. And Meredith did not end up in the Blue Cluster.

The Red Cluster

Instead of being a part of the original Blue Cluster, Meredith—at step #4—started the Red Cluster. Using the fictional chromosome browser, we can see she matches both Joe and Rebecca at segment #6. So Meredith, Joe, and Rebecca are the Red Cluster.

Consolidating Clusters

By using this fictional chromosome browser, we can see why Lori, Joe, Rebecca, Liam, and Meredith should all be in one cluster. They all came from one part of your tree! They are all related to Grandpa Fred! They only ended up in two different clusters because Lori and Meredith didn’t happen to inherit identical segments of DNA.

In this case, we also know they all belong in one cluster because we were able to identify these 5 matches and determined they were all related through Grandpa Fred’s line.

But, even without a browser or identifying these 5 matches, we can see the heavy overlap. When you see heavy overlap, you should probably combine the clusters!

A Chart with 4 Clusters

After consolidating the Blue and Red Clusters into one cluster, the Blue Cluster, we end up with 4 clusters. Each of these 4 clusters likely represents one of your 4 grandparent lines:

Do we need a chromosome browser?

In real life, we do not need to be able to see these chromosome segments to create these clusters! Instead, this example is meant to show you why people who are related to each other—like Lori and Meredith—might end up in different clusters. This happens because they don’t share segments of DNA. However, we can still combine clusters—and thus have Lori and Meredith in the same cluster—based on heavy overlap.

Questions?

I hope this helps! Please let me know if you have any questions or comments.