As we approach Christmas 2018, and given the massive push to have cheap DNA tests given out as gifts this season, it seems natural to finally write a series on how to make genealogical use of a DNA test you, or your loved one, may have just taken.
We’re going to start with the very basics on how DNA testing works, and walk through both how to leverage AncestryDNA to track down ancestors as well as using GEDmatch and other advanced tools to go even deeper.
Assuming you have a few weeks before the test results are in, here are a couple of things to learn and prepare before you dive into the matches.
- First, understand that while the commercials like to highlight the joys of learning your ethnicity, DNA testing raises serious issues that will likely come up as your journey progresses. You may uncover family relationships, both inside and outside of your family, that could have serious negative impacts on people. We’ve uncovered children born outside of marriages that were never known to the family, and we know of adopted children who were outed by tests where their parents had never told them. We wrote about an example of this last year (Dangers of DNA Testing).
- Second, they key to effectively making matches will be a good, solid family tree through the test subject’s 4x Great Grandparents. Most of your matches made will be through 3x or 4x GGP, and in a perfect world the match will also have a good tree so the link will be obvious. We can’t over state this, or stress it enough: your success/failure of matching DNA tests from unknown relatives will rely on the quality and depth of your tree. We’ve walked through how to build a good “quick and dirty” Public tree on Ancestry (Building a good Public Ancestry.com tree – Part One: sources, citations, facts, and proof), and the process would be about the same on other sites, many of which are free.
- It’s also important you have the tree available publicly…many of your interactions are going to be about exchanging trees to build a match. It’s ok if you have just a skeleton tree with basic information(names, date of birth/death, locations, children, etc.), but it will be key that you have something available publicly.
- Finally, we covered some of the same advice we’re going to cover here previously, it’s not a bad idea to go through the highlights to get an idea of what to expect from your results. (What to expect from your genealogical DNA results).
Basics of DNA
The main new term/concept you’ll need for effective Genealogical DNA research is a measure of distance: centimorgan (cM). Now, it’s not technically distance…but for all intents and purposes, it’s used as a measure of distance.
What does cM measure?
Centimorgan measures length of DNA strands. More specifically, it will be used to measure the length of matching DNA segments between your test and a test that is a genetic match. For example, you have roughly 6800 cM if you take all 22 chromosomes and strung them out end-to-end, and your matches will have varying levels of matching DNA, measured by centimorgans.
How do we use centimorgans to identify matches?
Since you get about 50% of your DNA from each parent, your DNA tests will match a test from your parents with about 3400 cM. You will match a Grandparent with about 1700 cM (50% of your parent’s 50%). The more cM you match someone, the closer a relative they are, and the more likely that you will confirm a match with them.
We’ll use both charts from ISOSG (The Shared cM Project table) and an interactive version of that chart from the DNA Painter site (Shared cM Interactive Tool), which both break down the average cM to expect with various relatives, and helps us identify where to look to establish a match. For example, if a match is 311cM then we can guess they match the person with the DNA test at around a 1st or 2nd cousin…which means our common ancestor is likely a Grandparent or Great Grandparent, which narrows down our search!
So, there’s the first part of this DNA journey. There’s a little homework while you wait for the test results, a basic understanding about how we’ll actually leverage the DNA to make matches, and why your basicGenealogy and a solid family tree will be key to this process. Next week, we’ll go over what to do when you first get your DNA results!