DNA Sibling Test:
Confirming Close Relationships

After a 26-year search, a DNA Sibling Test confirmed my place in my biological family.

This test was once the only option when one or more parents are not available for paternity testing. Yet it may be the least known and least understood form of DNA testing.

I’m hoping my experience will shed light on the test and guide people in similar situations.

If you haven’t already read the earlier parts of my adoption search story, you can follow these links:

Finding Birth Parents

Parental DNA Testing

Genealogy DNA Testing

NOTE: When I used this test in 2007, it was the only option for confirming close relationships. Since then, a far superior DNA test has emerged. Therefore, I no longer recommend this conventional sibling test. Read on to learn the details of my story. But be sure to see my current recommendations at the end of this page.

I had my birth father narrowed down to a man named Douglas--who knew my birth mother--and his four brothers who could have met her. A paternity test was not possible, because Douglas and his brothers were all dead. A Y-DNA test of their sons would not help, because all of them would probably have the same ancestral markers.

Understanding the DNA Sibling Test

I discovered the DNA sibling test, which some labs refer to as kinship testing or sibship testing. The test compares two or more potential siblings on genetic markers that we inherit randomly from both parents.

That randomness prevents this DNA sibling test from providing absolute “Yes” or “No” answers.

Instead, the lab calculates the probability that two people are full siblings or half siblings. They report this as a percentage, an index number, or both. The value depends on the number of matches and the rarity of those matches.

My Use of the DNA Sibling Test

I contacted one son from each of the five brothers, explained the test, and told them I would pay for testing. They all agreed to participate.

[I tested sons. But for this test you can recruit sons or daughters.]

Here’s how it should work:

• My birth father’s son would be a half-brother to me. We have the same father but different mothers. So roughly 25% of our DNA should match.

• My father’s brothers would be uncles to me. So their sons would be cousins. And we should have less in common genetically, about 12.5%.

• When the lab compares each person’s DNA to mine, one of them should stand out as a closer genetic match than the others. And that person’s father would be my father.

On the other hand, if these people are not close biological relatives, the test would show it.

Taking the Test

A DNA sibling test is not cheap. So I started by testing myself and three of the five potential siblings. If all three showed a low probability of relationship, I would conclude that I had the wrong family and stop.

I placed the order and provided names and home addresses. The lab shipped each of us a DNA testing kit with a DNA swab to collect cells inside our cheeks. After all the tests were in, I got my first test report.

[The form that each person sends in with their sample asks for a checkmark to indicate the subject’s race. Be sure everyone checks this. Some genes are more or less prevalent in certain races. So knowing the race increases the accuracy of the results.]

The first results were positive, so I ordered tests for the other two people. When those results arrived, I noticed that all of the reports only compared 11 markers. The lab I used advertised that they tested 16 markers.

Retesting with Blood

I learned that one of the 16 markers only indicates gender. So they calculate the index number from the other 15 markers. But for two of us, they could only extract 11 of those 15 markers. The other four had degraded.

This can occur from bacteria on food particles in the mouth. And some people don’t have enough saliva to provide a good sample from inside the cheek.

[If you do a cheek test, brush your teeth and rinse with mouthwash. Then wait an hour before collecting the sample.]

The lab technician recommended re-testing the two of us with a home blood test. When a cheek swab fails, a blood test is more likely to produce all good markers. They sent each of us a special card with some circles marked on it. I had to get some lancets like those used by diabetics. I pricked my finger and squeezed a couple drops of blood into the circles.

[In a DNA sibling test it’s important to test all possible markers. Be sure to count the number in your report and get new samples if necessary.]

Understanding the Reports

After much delay, I finally received a set of reports. The lab I used provides both percentages and index numbers. But the technicians I spoke with said to ignore the percentages, which can be misleading. The index numbers provide a more accurate comparison.

I received a separate report comparing me to each one of the others. For each genetic marker there are two values for each of us…one inherited from our mother and one from our father. Note that it’s not possible to tell which came from which parent.

For each marker they calculate a full-sibling index number and a half-sibling index number. When we don’t match, the number is 0.5 in the half-sibling column and 0.25 in the full sibling column. When we do match, the numbers are higher than that.

On my DNA sibling test reports I saw matches with half-sibling index numbers as low as .82 and as high as 6.06. Low index numbers mean that the value we both have is relatively common in our racial population. So that match doesn’t provide much evidence of relatedness. High numbers mean we matched with a value that is much rarer. So the chance of two unrelated people both having this value is small.

Finally, each report shows a combined index number for full and half-sibling situations. This number is actually the product of all 15 indices multiplied together. So all numbers below 1.0 pull down the total and all numbers above 1.0 raise it up.

My Results

Comparing me with the son of Douglas resulted in a half-sibling index number of 6.98. The other comparisons resulted in index numbers of 1.49, 1.26, 0.52, and 0.049.

Since we’re dealing in probabilities, the lab people are reluctant to draw any firm conclusions from a DNA sibling test. When pressed, more than one technician agreed on the following points:

• An index number of 1.0 means there is a 50% chance that the two people are siblings.

• An index number of 10 or greater is almost certainly conclusive.

• An index number between 1 and 10 that exceeds the next closest number by a factor of three is probably conclusive.

My index number of 6.98 with the son of Douglas was 4.7 times the next closest number of 1.49. When this result is combined with other evidence of their relationship, I feel completely confident that Douglas was indeed my father.

IMPORTANT: Better Tests Available Today

While a conventional sibling test worked well for me, I have heard from several people who received inconclusive results from a conventional DNA sibling test. Today there are two better alternatives for testing close relatives: Family Finder and Relative Finder.

See the pages in my Relationship Testing section for more information.

A Final Note on Physical Appearance

Having met many people in my new family, I can see a strong physical resemblance between me and several men. But I don’t look much like my father or my half brother. I asked the lab experts about this and they told me that none of the tested genes relate to physical appearance. Furthermore, physical appearance is a poor indicator of relatedness. As one technician stated…

“DNA trumps physical appearance.”

That’s something to remember when we’re tempted to make assumptions based on physical resemblance or a lack of it. Don’t guess. Do the tests.

For me, DNA testing uncovered the truth about my biological family and inspired me to create this web site.

For links to all four parts of my story…
Return from DNA Sibling Test to Adoption Search Intro

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