DNA Testing for Genealogy – Getting Started, Part Three

Posted August 1, 2012 by Geni | 26 Comments

We’re excited to bring to you a special guest series by genetic genealogist CeCe Moore. Some of you may recognize her from her blog YourGeneticGenealogist.com. For the next few weeks, she’ll be providing a great overview about DNA testing for genealogy. Enjoy!

This week we are finally going to discuss my favorite type of genetic testing for genealogy – autosomal DNA. For the past two weeks we have covered DNA tests that are solely informative of one ancestral line – direct paternal (Y-DNA) and direct maternal (mtDNA). The great news about autosomal DNA (atDNA) testing is that there is potential to find valuable and meaningful information about any of your ancestral lines.

This is because all of us, male and female, inherit our autosomal DNA from our parents. Fifty percent of our atDNA comes from our mother and 50% comes from our father. Since our parents each received 50% of their atDNA from each of their parents, it follows that we inherited about 25% of our atDNA from each of our grandparents. This percentage, which is somewhat variable, is cut in half with each generation as we go further up our family tree. As you can see on the chart below, we inherit about 12.5% of our atDNA from each great grandparent and about 6.25% from each of our 2nd great grandparents.

The amount of autosomal DNA inherited from our ancestors on average (Credit: Angela Cone)

The Basics

Without getting too technical, all healthy human cells possess 23 pairs of chromosomes. We receive one set of these 23 chromosomes from each of our parents. Chromosomes 1-22 are called the autosomes. The 23rd pair is what determines gender. One X chromosome from mom and one Y chromosome from dad, results in a male and one X chromosome from each results in a female. The autosomes are the focus of the type of DNA test we are discussing today.

When you submit a DNA sample for an autosomal DNA test, the testing company will compare your atDNA to that of everyone else in their database, searching for people who have significant stretches or “segments” of atDNA in common with you. If two people share enough atDNA, then it follows that they must have inherited it from a relatively recent common ancestor. Based on this principle, you will receive a list of matches with a prediction of how closely related the testing company estimates you are to each. Generally, the more atDNA two people have in common, the closer they are related and, thus, the more recent their common ancestor(s).

Since autosomal DNA, unlike Y-DNA and mtDNA, undergoes mixing (recombination) every generation, its usefulness quickly dissipates for genealogical research more than five or six generations back. Because of this, it is advisable to test the oldest living generation whenever possible. This isn’t to say that you won’t occasionally find an atDNA match with a cousin more distant than third or fourth, but the odds become increasingly smaller, so it is far from guaranteed.

What can atDNA testing do for me?

In my opinion, there are three important aspects of atDNA testing for a genealogist. The first is finding previously unknown cousins who can either help you get beyond a genealogical brick wall or provide you with other new information about your shared ancestors. The second is testing known relatives to confirm relationships and, in doing so, identifying DNA from specific ancestors. The third is receiving a percentage breakdown of your “ethnicity”.  The first two are within the scope of this post and the third will be discussed next week.

Since we inherit atDNA from all of our relatively recent ancestors, there is potential to discover a match in the database who shares descent from any of our ancestral lines. This includes those that are currently brick walled within the last couple hundred years or so. If, for instance, you have a brick wall at the great-great grandparent level in Sidney, Iowa with the surname Travis and you find a predicted third cousin match with a person who has ancestors with the Travis surname in Sidney, you may be able to gather enough information from your match and/or this nudge in the right direction to demolish that brick wall. This may even hold true for an unknown maiden name of one of your female ancestors if you are lucky enough to find a strong match in the right geographic area.

One of the most meaningful aspects of atDNA testing is confirming genetic descent from our ancestors that, so far, have only existed in our present-day lives on paper. Each time I confirm a common ancestor with one of my matches, I have taken a step toward authenticating that branch of my family tree. Let’s face it, no matter how good the documentation, it is simply impossible to know for sure that our ancestors are always who they are supposed to be on paper. However, with several DNA matches who can document descent from the same common ancestor(s), we can feel quite confident that our research on that part of our tree is correct.

My 22 autosomal chromosome pairs in gray, paternal and maternal genetic contributions top and bottom

I have been testing my known cousins for some time in order to “map” my chromosomes to my great grandparents. For example, if I test my known Proctor second cousin (who shares no other ancestral lines with me), then I know that any shared atDNA that we possess is inherited from our common great grandparents. A genealogist can systematically test known relatives to determine which portions of their chromosomes were inherited from which of their ancestors. This process can also be used to confirm traditional research as described above. So far, I have tested multiple cousins from each of my ancestral lines and confirmed that my great grandparents are indeed who I thought they were. In the image to the right, you can see the beginnings of my “chromosome map,” color-coded to illustrate which portions of my autosomal DNA were inherited from whom. 

New Possibilities

I have been interested in DNA testing for genealogy since 2002, but I didn’t get really excited about it until 2009 when 23andMe introduced Relative Finder, the first version of autosomal DNA matching for genealogy, and Family Tree DNA soon followed with their version, Family Finder. I was immediately hooked. What I really love about atDNA testing is the possibilities, the almost unlimited potential for discovery. For the dedicated genealogist venturing into atDNA testing, with perseverance, there is much to be gained.

A few examples of discoveries made from atDNA testing:

  1. I found that I have distant Jewish ancestry about which I might never have known without atDNA testing. My colleagues and friends Dr. Tim Janzen and Andrea Badger confirmed Jewish descent as well.
  2. Tim also confirmed Native American ancestry in his family.
  3. My brother-in-law learned of African ancestry and, as a result, uncovered a direct descent from Thomas Jefferson.
  4. Adoptees have discovered close family members and reunited with birth parents.

Although atDNA testing is full of promise, it is not recommended for those who desire quick and easy answers. Contrary to some people’s expectations, the testing company will not populate your family tree with your missing ancestors (maybe someday). However, I believe that atDNA research is a perfect fit for the devoted genealogist with an already well-researched family tree. Many of us jump for joy when we discover a new tidbit about an ancestor after years of searching.  Just imagine how meaningful it is when presented with living evidence of your ancestors’ existence that is inside of you? If you give it a chance and are willing to work hard at it, autosomal DNA testing can be an incredibly rewarding experience.

Next week, this series will conclude with a discussion of tests that provide percentages of “ethnicity” and a summary of what I hope you have learned from this series.

(Update: Read part 4 here)

CeCe Moore is a professional genetic genealogist and writes the popular blog Your Genetic Genealogist. She is the Southern California Regional Coordinator for theInternational Society of Genetic Genealogy and the administrator of the organization’sDNA Newbie Yahoo Group. CeCe serves as an “Ancestry Ambassador” to 23andMeand on the advisory board of the Mixed Roots Foundation and is a member of Mensa. Her favorite genetic experiment is her seven-year old son, Nicolas.