What do my siblingship DNA test results mean?

One of the most common searches that people find my blog through is siblingship DNA tests. In the past few months I have received MANY emails from adoptees and donor-conceived adults who have gone through a DNA test with a possible sibling - only to receive results they don't really understand. Some simply do not understand what their results mean. Others are confused as to why they are inconclusive. Many others think that there's still a possibly of them being siblings when in reality the results are VERY negative.

Most just want the peace of mind of knowing if they are siblings or not. For most of these people I am able to give them some concrete answers or lead them in a direction to provide them better results.

This shows me that most DNA testing companies are doing a less than satisfactory job at explaining what an individual's results mean in plain language --- a catch that many in the medical field struggle with. Medical information to the layman means nothing if they cannot comprehend what they are being told or reading.

So I thought, rather than talking about DNA testing in general or what all the different numbers mean, I wanted to discuss negative and inconclusive results as well as postives and provide some examples of these types of results as well as how DNA testing companies word results (that make them indecipherable to those without background knowledge).

Below are some examples of actual DNA tests (names have been removed to protect the individuals) with some explanation as to what they mean.

Example #1: Conclusive sibling match w/ both mothers tested.

Below are results of a siblingship DNA test with two alleged siblings and both their mothers that is clearly a conclusive match.

In the above example the two alleged siblings and both their mothers were tested. When both mothers are tested it allows the analyzers to determine which allele at a specific marker came from the alleged shared biological father. If the two alleged siblings share this allele it increases the individual SI for that marker. The results are in the yellow box below. This is very obviously a sibling match as they are 99.8% probability of being siblings. The CSI (combined siblingship index) is over 500, signifying a very strong conclusive result. No questions asked, these two individuals are most definitely half-siblings.

However, as I show below, even in the case where two individuals are most definitely siblings in one test, take away the mothers and those results change dramatically!!

Example #2: Inconclusive sibling test w/ neither mother tested.

If we took the same two siblings in Example #1, but neither of their moms were tested, here's what we would have...

The same two individuals that in Example #1 were surely siblings, these results are questionable, and would be considered inconclusive. They cannot be deemed negative because they are between 0.5 and 1.0 (although nearly on the brink of going below 0.5). Their probability is only at 33% but their odds are 1.95 to 1. That is....they are 1.95 times more likely of NOT being half-siblings than being half-siblings. These odds, while still suggesting inconclusive negative results, are still fairly low.

Odds that are more like 20 to 1, 50 to 1, 100 to 1, are more significantly negative results. A result received like in Example #2 suggests to me that further testing should be done.....especially if one or both mothers were not tested!!!! If both mothers ARE tested, I would suggest a second panel of STR markers (autosomal markers like the ones above...most DNA testing companies can do extra if you request them) or either X or Y chromosome testing (if the two alleged siblings are of the same sex). The latter is preferable for same-sex alleged sibling pairs.

Example #3: Inconclusive sibling test w/ one mother tested.

Here is another example of an inconclusive result. In this case only one mother has been tested (as can be often the case with older offspring and adoptees where one mother may have already passed away).

The results suggest relatedness, as in the CSI is over 1.0 and the probability is over 50%, however it is only over the threshold by a smidgen.

The two examples I showed above provide reason as to why I STRONGLY suggest at least one mother to be tested!!! I know there are many siblingship tests that are found to be inconclusive or downright negative when in fact they may actually be siblings, simply because neither mother is tested. In Example #2, these two individuals, if one of the mothers were tested, their results would have been at 92% probability - enough for the US Courts, but would not have been successful with UKDL's stringent 99% cut-off. In Example #3, had it been possible (one mother was deceased), a further test taking into account Sibling 2's mother could possibly have provided these two individuals the peace of mind of knowing that they were or were not truly siblings. I suggested further STR marker tests for these two.

It's this probability and odds that confuse many people, and are what constitute the majority of questions I receive. Remember: Siblingship tests are based on a prior probability of 50%. This means that 50% probability is the starting point and and results that lean towards being positive will raise that probability above 50% and results that lean towards being negative will lower that probability below 50%.

Some people are confused and this that a probability of say 20% means the probability of two individuals being siblings picked from random - i.e. 20% with zero as the starting point. This makes a 20% probability sound relatively supportive. However, when a siblingship test is being conducted, they are already under the assumption of being possibly related and not random individuals, thus the prior probability is 50% (there's a 50-50 chance that they are or are not related). Therefore, a probability of 20% is NOT good.

Example #4: Conclusive negative sibling test w/ both mothers tested.

Below is a conclusive negative test, with not questions asked these two individuals are not related at all.

In this example these two individuals are conclusively not siblings. With results like these it would be nearly impossible for further testing to provide anything but similar negative results. The combined siblingship index is waaaayyyy below 1.0, and the odds are over 1,000.

I hope these examples and their explanations have helped many of my readers and those that find my blog for these questions, to better understand your results, and have more confidence in what they mean. As always, please email me if you have any further questions or would like me to take a look at any results and provide a better understanding into what they mean.