I was first introduced to this concept via a cute comic strip. The crux of the problem is that people may share more than one line of genetic descent. If Alice and Bob are related, a DNA test will show that. But the relationship may be complicated: they can have multiple shared ancestors. Many combinations are possible. What's more, Alice and Bob may know of one shared ancestor, but their DNA connection might be through an ancestor that is unknown to one or both of them!
|Congratulations, you found a DNA match.|
You still haven't proved anything.
| An example of shared segments in|
FTDNA's chromosome browser
There exists a fantastic third-party website called GEDMatch that aims to make all this easier. It accepts raw data from any of the aforementioned services and runs its own comparisons and triangulations. Unfortunately, as I understand it, the site is run by volunteers on a shoestring budget who are currently swamped by all the incoming data: DNA comparisons are computationally intensive. The site is not currently accepting new data, but is projected to start again in August. I encourage everyone to make use of this tool when it becomes available and then to DONATE so they can continue to function. I'm glad to see that they stepped up where the large companies didn't.