A DNA relationship test can confirm that two people are related.
For example, an adoptee finds her presumed birth family. But the birth parent is not available for a paternity test. The adoptee can confirm the relationship by testing a potential half sibling.
That’s what I did in my own adoption search.
Another example occurs when a child’s alleged father dies without a paternity test. The man’s parents may wish to know if they are the child’s biological grandparents.
My Guide to DNA Testing
In addition to exploring this web site, I suggest you dowload my eBook: Guide to DNA Testing.
I describe and compare the major test types. Then I explain their purposes, strengths and limitations in a unique table that you will not find anywhere else.
This Guide provides the high level overview you need to put the various tests in perspective.
There are two types of DNA relationship testing. Both methods depend on the principle that closely related people have more DNA in common than unrelated people.
Also called sibling testing, this is the old-technology DNA test to show that two people are related.
The test compares two or more potential relatives by counting Short Tandem Repeats (STRs) on a small set of autosomal markers. Autosomal markers reflect the genetic contribution of both parents.
This type of test cannot be as conclusive as a paternity test. That’s because there is no way to tell which marker value came from which parent.
At best, it can estimate the probability that two people have a given relationship. At worst, it can provide misleading results.
Now that better tests are available, I no longer recommend this type of test.
See DNA Kinship Testing for more about this type of test.
An alternative test type emerged in late 2009. That’s when 23andMe introduced their Relative Finder feature (now called DNA Relatives). Early in 2010 Family Tree DNA followed with their Family Finder.
Both are examples of Autosomal DNA Testing. This type of test (based on SNPs instead of STRs) is far more conclusive than traditional Kinship Testing. It checks more than 700,000 markers and actually MEASURES the amount of DNA that two people have in common.
No more guessing. No more results that just say "Maybe."
By measuring the number and length of common DNA segments, these tools can tell if you are full siblings, half siblings, first cousins etc.
NOTE: A third test of this type, AncestryDNA, is now available. Unfortunately, it does not distinguish close relationships as well as Family Finder and 23andMe. So I don't recommend it for testing close relatives.
See my page on Relative Finder as a Kinship Test for more details on this use of the 23andMe test.
Also see Autosomal DNA Comparison for a more detailed comparison of the two tests.