DNA Kinship Testing
on Siblings & Others

DNA Kinship Testing is a broader term for what we often call sibling testing. You may see additional coined terms such as “siblingship” or “sibship” testing.

Basic DNA kinship testing compares two people on a small set of autosomal genetic markers. You inherit these autosomal markers randomly from both parents

Your values on these markers may indicate a high or low probability of a close relationship.

Many Labs Fall Short

If you choose a lab that’s AABB-accredited, you can assume they’ll do the test right. But many labs still promise more than they can deliver. Or they do a terrible job of interpreting and reporting results.

I know this because many people have contacted me through my Feedback Form regarding their DNA kinship testing results.

Counting Matches Is Not Enough

Some labs only report the number of matches, e.g. 9 of 15 markers. But some marker values (alleles) are rarer than others. Those who share rare marker values have a higher probability of being related than those whose marker values are relatively common.

Good labs will factor rarity into their calculations. Those who do will typically report the results as index numbers or probabilities.

Beware of Premature Conclusions

I’ve seen reports where a lab concludes—from the autosomal test alone—that two people are related or not related.

Unlike a paternity test, DNA kinship testing does not produce obvious Yes/No answers. Just testing a few autosomal markers cannot, by itself, prove or disprove a relationship.

My Recommended Lab for This Test

When I did my DNA sibling test in 2007, I was lucky enough to choose a lab that reported the results properly. Still, their phone support was weak and getting good answers to my questions before and after the test required extraordinary persistence on my part.

Today I can only recommend one lab for DNA kinship testing and that is Family Tree DNA. Yes, they are AABB-accredited. And they offer both 15-marker and 24-marker autosomal tests for siblingship. But that’s just the beginning.

From Probabilities to Proof

Family Tree DNA can often go beyond probabilities and actually prove relationships. Their goal for every customer is to provide DNA evidence that could, if necessary, hold up in a court of law.

To explain how they can do that, let’s use a common example: two people trying to prove they share the same father.

Run More Tests

If the two potential siblings are both male, Family Tree DNA can also check their Y chromosomes. A match there can prove they share the same father, as long as they can exclude another connection in the direct paternal line.

If the two potential siblings are both female, Family Tree DNA can check their X chromosomes. A man has only one X chromosome, which he must pass on to each daughter.

Therefore, if they really do share the same father, both sisters will have at least one value in common on all tested markers.

Unfortunately, if the potential siblings are a male and a female, there is no simple supplemental test to prove relatedness.

Test More People

Another path to increased confidence in the results is to test more people with known relationships to the two primary test subjects.

If that’s necessary in your case, Family Tree DNA will recommend additional test subjects.

When I did my test with another lab, I designed my own test plan. I tested the son of my suspected father and a son from each of the man's four brothers. My half brother clearly stood out from my half cousins.

See my DNA Sibling Test page for more on my personal use of DNA kinship testing.

How to Order

The ideal set of tests and subjects will vary with the specifics of each situation. Therefore, I strongly suggest that you do NOT attempt to select the tests yourself.

Go to the Family Tree DNA web site, click on Products, then scroll all the way to the bottom of the page. Under Specialty Testing Services click one of the email links and describe your situation.

A specialist in DNA kinship testing will get back to you with the suggested tests and a quotation.

If they can’t devise a suitable test plan for your situation, Family Tree DNA’s adviser will tell you up front. So I suggest you contact them first before making any decisions on relationship testing.

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