Thursday, January 4, 2018

What is the big deal about a Chromosome Browser?


This is a continuation of my previous post Getting more from Family Tree DNA Family Finder

In order to understand the importance of a chromosome browser, we need to first review what I previously wrote about autosomal DNA and how it is inherited.


Autosomal DNA 


Several companies offer autosomal DNA tests. The four most popular companies are Ancestry DNA, 23andMe, Family Tree DNA, and MyHeritage.   

You inherited your DNA from both of your parents, and you can use the autosomal DNA tests from any of these companies to trace multiple ancestral lines. However, you are carrying less DNA (and maybe none at all) of more distant ancestors because of the way autosomal DNA is inherited.

You inherit 50% of your DNA from each parent. Of course, this also means that 50% of the DNA of each parent is lost unless you can test the parent or test siblings or their descendants. If you have siblings, they inherited 50% of their DNA from each of your parents, but not the same 50% unless they are identical twins. 

Here is an example of what could have been inherited by four siblings. This shows a tiny portion of the DNA of a father, mother, and what was passed down to each of their four children.


autosomal inheritance



How can I see a comparison like this for my own DNA?


You can see inherited DNA segments by using a chromosome browser. The chromosome browser will compare your DNA to the DNA of another person and let you see which DNA segments are shared between the two of you. 

Some companies provide a chromosome browser to let you do this. For example, Family Tree DNA and 23andMe both allow you to prove relationships by viewing your DNA in a chromosome browser. Ancestry DNA and MyHeritage do not provide chromosome browsers. [Update: MyHeritage added a chromosome browser on 11 Jan 2018.] However, no matter what company you used, you can download your results and upload them to a service called GEDmatch. We will see how to use GEDmatch in my next post, but for now, let's use Family Tree DNA's chromosome browser to see why it is such a powerful tool.
  

Why is a chromosome browser important?


DNA results without a chromosome browser is like a genealogy without documentation. When you look at someone's family tree and there is no documentation to prove it, all you have is some hints of possible relationships. There is an old saying that "Genealogy without documentation is mythology." The same applies to your DNA.

If I tell you, "I've compared our DNA, and I know that we both inherited DNA from John Smith who was born in 1852," you might ask, "Can you prove that?"  Without a chromosome browser, I would have to say, "You'll just have to take my word for it." You would have a "hint" from me that we might share DNA from John Smith. But we don't want to take the word of any genealogist or DNA company. We want proof! A chromosome browser and some documented genealogy can help provide the proof we need.


Chromosome browsers for close family relationships


My mother was separated from her family when she was six years old. Her three siblings went to different families, and none of them knew what happened to the others. However, after they had been separated for 52 years, I was able to find all of my mother's family. We discovered that she also had half siblings. 

My mother and her younger sister didn't look anything alike, and they had very different personalities. For many years they suspected that they were not full siblings. Finally, a few years ago they did DNA testing. My mother and her sister tested with 23andMe, Family Tree DNA, and Ancestry DNA. One of their half brothers tested with Family Tree DNA.

At Family Tree DNA, you will have a list of Family Finder matches.  Here is what it looked like for one of the sisters:


Family Finder matches


Looking at the Relationship Range column, you can see that she has a match for a woman who appears to be a full sibling, and one man who appears to be a half sibling. This column is estimating the relationship based on the amount of shared DNA. Notice, however, that the Relationship Range indicates that the full sibling could also be a half sibling. So next we looked at the chromosome browser to prove their relationship.


Click on the box next to the half-brother's name. Then go to the top of the page and click Chromosome Browser.

 
Family Finder chromosome browser


Here is what the chromosome browser showed. All of the DNA segments highlighted in orange are shared between the sister and her half-brother.


half-sibling chromosome browser


Now go through the same process with the sister. Click the box next to the name of the sister, and then click Chromosome Browser.  Here is the common DNA shared by two full sisters.


Full siblings shared DNA

 
You can click next to both names to compare two people in the chromosome browser. We clicked the sister first, then the brother. Here the orange color indicates the DNA shared with the first name that was clicked [the sister]. The blue is the DNA shared with the second person [the brother]. The blue color is not an indication that the second person is a male.



chromosome browser


My mother and her sister still weren't convinced that they were full siblings. But the chromosome browser from 23andMe settled it.



full siblings shared DNA


The segments in light blue are shared by both sisters, and the common DNA was inherited from one of their parents. The segments in dark blue were shared by both sisters and inherited from both parents. The white segments show where each sister inherited something different from both parents, and so they don't have any common DNA in those places.

This very clearly shows how important it is to test as many people as possible if you want to find more about your ancestry. All of those white and light blue spaces indicate that the sisters did not inherit identical DNA in those locations. So without testing both sisters, look how much of their parents' DNA would have been lost!

The chromosome browser from GEDmatch showed the same relationship.  These two women are definitely full siblings.


GEDmatch chromosome browser

This is the key to the GEDmatch chart. The green "Full Match" means that the DNA was inherited from both sides of the family.


GEDmatch chromosome browser


 Chromosome browsers for the first five generations


It is commonly stated that autosomal DNA is good for tracing ancestors within your first five generations. This is because we inherited 50% of our DNA from one parent, about 25% from a grandparent, and so on. When we get to more distant generations we may not have inherited much, or even anything at all. 

If you want to see DNA segments that you may have inherited from a particular ancestor, you can search for the name of the ancestor, compare family trees, and compare DNA in the chromosome browser.

Go to the Family Finder section of your FTDNA account, and click matches.


Family Finder matches


Search for your surname of interest:


suname search



Click the family tree symbol to see your match's family tree: 


see Family Finder family tree


When you find a common ancestor, check the box next to the person's name to compare in the chromosome browser:



Family Finder match


Go to the top of the page and click Chromosome Browser:


compare dna


Here's what the chromosome browser shows for two third cousins. We know who their common ancestors are--the husband was born in 1803, and the wife was born in 1821. We don't know which of these segments came from which ancestor, but we know that these people are definitely related.


chromosome browser


However, matching DNA does not prove that we inherited it from any particular ancestor. We can add more evidence when we add another 3rd cousin who descended from the same ancestral couple:


compare DNA


The two matching relatives share very different DNA segments with my uncle, but they descended from the same family. Did all of these DNA segments come from the same ancestors? We can only know by comparing more descendants. This is, again, why it is so important to test as many relatives as possible. We did not all inherit the same DNA from our ancestors.


DNA for distant relatives


You may not have any DNA from some distant ancestors, but all of your DNA has been passed through time, and some common DNA segments can be much older than five generations.

Here is a Family Finder chromosome browser image showing matching segments from four of my uncle's matches. All five of these people descend from different children of a common ancestor born in 1722.



distant relative DNA match

We can see that they all share a small segment on Chromosome 5. Does this mean that I've proven the relationship? No. These are small segments, and small segments are often coincidental. For an excellent explanation of this, see Blaine Bettinger's post A Small Segment Roundup. We have to be very careful when assuming that small DNA segments are "proof" of common ancestry. 



So why is a Chromosome Browser such a big deal?


Just because you and another person have the same ancestor in your tree, it does not mean that you inherited your common DNA from that ancestor. But you can see where you have matching DNA segments and compare those DNA segments with those of other people who descend from the same ancestor.  

When you have tested three people who descend from three different children of the common ancestor, you are on your way to proving the relationship. This is called triangulation, and we will examine this further in the two upcoming blog posts. 

First, we will see how to make matches with people who tested with different DNA companies. Then, with information from chromosome browsers, we will start reconstructing our ancestors' DNA. Seeing that in action will blow you away.

Up next:  Getting started with GEDmatch

and How to use GEDmatch - Part 1

and How to use GEDmatch - Part 2

and: Using DNA Painter to reconstruct your ancestor's DNA You will not want to miss this one!


______________________________________________________________________
Standard Disclosure
This standard disclosure appears at the bottom of articles in compliance with FTC Guidelines. I evaluate products of DNA testing companies and show how to use them. This article is about Family Tree DNA. If you wish to purchase one of their products, and you click through the link on the sidebar of this blog, I receive a small contribution if you make a purchase. Clicking through the link does not affect the price you pay. Thank you for clicking the link!



5 comments:

Jason Lee said...

Crucial information. Thank you!

Roger Peak said...

Your initial illustration shows the four nucleotide bases, yet you do not mention them in your article. How can we use a chromosome browser to understand the changes in those nucleotide bases?

Linda Jonas said...

Roger, the first image is not showing nucleotide changes [mutations] in anyone's DNA. For each person, the image is showing two copies of each chromosome--one that was inherited from the mother, and one from the father. The good news is that we don't need to understand any science whatsoever to find more about our ancestry. We just need to compare DNA segments.

Grace Compton said...

You make it very understandable, better than any thing else I have read. Thank you

Linda Jonas said...

Grace, Thank you for your kind words!