Autosomal DNA testing, for genealogical purposes, was first offered to the public earlier this year. We are still in the beginning stages of learning how to best apply it to further our family history research. It is predicted that such testing may be able to identify some relationships as far back as fifth cousins. In this kind of testing men and women are on a level playing field. One thing is becoming clear in the early days of testing. This kind of testing seems to rely even more heavily on well researched family trees than have the earlier types of testing. No longer can one assume that at match lies on the paternal line as is the case in y-chromosome testing. Likewise, one cannot assume that a match lies on the maternal line as is the case in mitochondrial testing. It can be on either of those lines or on any line in between.
If autosomal testing turns out to live up to the promotional promises of testing labs, most of us will have a hard time taking full advantage of the power we are being offered. Rather than being the “magic bullet” that by itself reveals our family trees, it re-emphasizes the need for traditional genealogy research. Experienced genetic genealogist Larry Vick stated it this way, “Whether we use Relative Finder, Family Finder, or both most of us need to do a much better job on our pedigrees to get more value from either test. I plan to spend a week at the Family History Library this summer beefing up my pedigree."
What is the probability that patrons will share enough DNA with a relative that it will be detected by autosomal testing? If you are related within five generations (3rd or more recent cousins) then autosomal is almost sure to detect your relationship. Testing may also detect many 4th cousins and a few 5th and more distant cousins. Early experience with autosomal testing by both FTDNA and 23andMe indicate the following chances of finding a match based on a biological relationship within the last few generations:
So if someone is a 4th cousin or closer, there is a good chance they will be detected in autosomal testing. Farther back detection of matches is a long shot but possible. The main problem is that even those of us with well researched pedigree charts rarely know who all our 32 great-great-great grandparents are. They would be the ancestors we have in common with a potential 4th cousin. In addition, the potential 4th cousin would also have to have documented who their 32 great-great-great grandparents were as well. Beyond this generation some matches will occur. In fact many will show up in the results of an autosomal test because we have many more potential living cousins as we go back a further generation of two. However, the likelihood of identifying a connecting ancestor becomes remote.
[To review previous posts on DNA in this series, click on the Label: "DNA Testing" just below this post and then scroll down through the series.]