Lesson 10: More with atDNA Matches

For the genealogist matching atDNA is really what it's all about. ANCESTRY is the only company that does not allow you to see where you match segments. FTDNA and 23andMe both show matching segments. Although ANCESTRY lacks the ability to see matching segments, where ANCESTRY really shines is in its tree matching ability and its extensive database of easy to view family trees. Let's take a look. Hint: just click on image for a closer view.:

Shown are my top four matches (closest or most shared DNA) at ANCESTRY. You will see I have three 3rd cousin matches and one 4th cousin shown above. In reality I have nearly one hundred 4th cousin matches and several thousand distant cousins on ANCESTRY. The filters on the top right allow you to sort by those matches that have "Hints," those that you have not looked at yet or "New," and those for which you have placed a star of "Starred." You can also use the "search records" feature to search by surname or location. I have scrubbed my matches' names but you will see there is a predicted range and a "Confidence" level for each match. Also there is the last date of Log-on "Last logged on" (which should be taken with a grain of salt as these are not always accurate, especially for those who stay logged on all the time). In the middle is an icon which denotes they have a tree and how many people are listed in the tree. In the above example there is a leaf icon. These are like pots of gold at the end of the rainbow. Here is my top match and our "Shared Ancestor Hint."

So this shows how my predicted 3rd cousin is indeed my 3rd cousin! We share the same set of great-great-grandparents. This happens to be a cousin I knew about but did not know she had tested. My next 3rd cousin on the match list is actually my fourth cousin however the range says 3rd-4th cousin. The third on my match list is a person whose tree is not complete enough for us to make the connection. The fourth on my match list is a predicted 4th cousin who is indeed my fourth cousin and I was able to make the connection for them based on information I had in my tree even though our surnames were spelled differently (Stuart and Stewart). The "Shared Ancestor Hint" only works when the algorithm used can make matches in both trees. This function works best if you:
  • Use standardized place names
  • Have a good size tree (it can't match what isn't there)
  • Include siblings and spouses of your direct ancestors and their children
  • Give an approximate birth year and place for all people in your tree
Obviously this feature won't work for those just starting out or those who do not know their parents or grandparents. The DNA matching works regardless of your online tree or its accuracy.

FTDNA and 23andMe include the ability to run comparisons based on segment matching. Keeping track of segments is called "chromosome mapping." Let's take a look at the advantages of being able to look at the matching segments. Here is a view from FTDNA's "Family Finder's Chromosome Browser."

I have selected 4 matches to compare DNA with. The orange and green segments represent a brother and sister who are my maternal first cousins. The small magenta segment is someone who matches one of my cousins but not the other on this segment. The blue segment represents someone who matches me via my father's side. There is an option to download these matches in EXCEL (CSV) format or in a table. This is where you would get the data to save in your "Chromosome Mapping File" (a place to store all match data). FTDNA allows you to look at up to five of your matches and compare them with you. One feature FTDNA has that is not available at 23andMe is the ability to reduce the size of matching segments all the way down to 1 cM. This can be useful in seeing how a longer segment may have broken apart into several pieces.

23andMe's chromosome browser is located under the tab "My Results" then "Ancestry Overview" then "Ancestry Tools" and finally "Ancestry Inheritance: Advanced" (not exactly easy to find but one of my favorite tools at 23andMe).

If you are lucky enough to test multiple family members you can see how random inheritance works by comparing segments. And sometimes you are surprised.
 The following "Comparison Chromosome Map"  illustrates how my maternal first cousin matches me at 23andMe and how two of my children match this same cousin.

I have blocked out my cousin's name. The dark blue bars represent where my cousin and I share a segment of DNA. Since we are first cousins we share a set of grandparents which is where our shared DNA comes from. The green and lighter blue represent matching segments of my children with my cousin. You will note that my children did not inherit all the segments that I share with my cousin. Sometimes they inherit the whole segment, sometimes it is smaller and sometimes not at all. Sometimes like on Chromosome 15 only J Wheaton inherited the segment that I share with my cousin but he only inherited half if it. See the small segment we all share towards the beginning of Chromosome 1. Sometimes this is all that might remain when my children's great grand-children look at the DNA retained from what are their great grandparents represented here. The randomness of the inheritance is what confounds researchers and DNA algorithms designed to tell you what the likely relationship of your match is. The closer the relationship the more likely the program will guess correctly. After about 3rd cousins the amount of retained DNA is so variable that the range is quite wide.

There are two anomalies to note on the above comparison: on chromosome 17 and on the X my children share a segment with my first cousin that I don't. How can that be? In the absence of other information one might discount these segments as IBS (Identical By State), however I have also tested my husband (their father) and he matches my first cousin on these segments! So the majority of the segments my children share with their first cousin once removed are through me---however two segments are through their father. This means my children are related to my first cousin through me and through their father! See what I mean about surprises?

These surprises are why the serious genetic genealogist wants to be able to see the matching segments, and track where they came from by collecting information on all of one's matches in a spreadsheet and doing careful analysis to make sure segments come from the ancestors we think they do. It is not possible to do this directly at ANCESTRY which is considered by some to be a major shortcoming. You can do a work-around by downloading your RAW DNA and uploading it to FTDNA ($69) or GEDMATCH (free) where you can use their tools to find out the matching segments and then use a spreadsheet to track segments.

Unlike FTDNA who includes all your matches in your list in their chromosome browser, 23andMe restricts their tool to those who have accepted a share request. (You must invite matches to share genomes at 23andMe and they must accept in order to see where you match.) This is a distinct disadvantage with 23andMe. An advantage of 23andMe is 
anyone you are sharing with can  be compared with any three others with whom you shareAt FTDNA you are always in the primary position with you being compared with several individuals. The ability to compare your matches with each other at 23andMe allows greater refinement of your chromosome mapping.

I hope that it is becoming clearer why I recommend using all three companies for the serious genetic genealogists. They all have their place in the genealogist's tool box.

Additional Resources:

Shared DNA Cheat Sheet by Kelly Wheaton see file at bottom of page

Genetic Genealogy and the Single Segment by Steve Mount

Cousin Statistics ISOGG

The atDNA Gamble by Judy G. Russell

The Common Mismatch by Judy G Russell

Why Don't I match My First Cousin? by Roberta Estes

Why Autosomal DNA test Results Are Significantly Different for Ashkenazi Jews By Jeffrey Mark Paull et al

Please see next lesson for links to in-depth articles on how to use various tools at the different test sites.

Content copyright 2013. All rights reserved.

LESSON 11: Deeper Exploration by Subject


Kelly Wheaton,
Feb 22, 2017, 9:09 AM