Sibling DNA testing is definitely different from paternity DNA testing in many ways. Ours is different from the rest because we test more markers than any other lab. More on that later. Probably the most distinct difference is the fact that two full siblings do not have to share a common DNA type at every marker. This makes the comparison purely statistical. Calculating the likelihood that two siblings are full siblings, half siblings or unrelated relies on how many markers they share in common and how rare the DNA type they share is in the population.
For many years the standard sibling DNA testing consisted of testing 16 markers with one being a gender marker. The 15 data points were plugged in to a complicated statistical formula and determined a percentage for full vs. half siblings and half siblings vs. unrelated. Not really “the good old days” for sibling DNA testing. An uncomfortable percentage of the sibling cases turned out to be inconclusive which was always a difficult explanation. It was as disappointing to me as it was the client. DNA testing siblings with 15 markers amounted to buying a new flat screen TV with black and white picture, not good!. While some labs today still test these same 15 markers clearly our clients needed an answer and I needed a better test to provide them with one.
In 2010, we developed new 40+ marker test for sibling and other non-paternity cases. This new test proved to dramatically reduce the number of inconclusive results and pushed most cases to +99.9%. It also highlighted issues with only testing 15 markers that we knew existed but that no one wanted to discuss. Mainly, an issue known as “misclassification”. Fancy way of saying “wrong answer!”. In 2011, We made the decision to only test non-paternity cases using our new 40+ marker test because the results spoke for themselves.
Paternity DNA testing is still the best test to determine who the true biological father is but the 40+ marker test has certainly made my job easier. At least now the results seem to be in full color.