I came across this in Google while searching about siblingship DNA testing. I thought that I might be able to test myself, my sister and my mom to see if my sister and I shared the same father. From what I've read, I will have to get BOTH my dad's & uncle's DNA to be 100% conclusive, even if they're not 'identical twins'. jill
Ask a Geneticist
by Dr. Barry Starr, Stanford University
I have heard that paternity tests can be wrong if the real dad is related to the tested man. Is this true?
September 29, 2010
Yes it is true. If two men are closely related, a standard paternity test might not be able to tell them apart. Which means in some situations, the wrong man could be identified as the father.
Imagine two brothers who might be the father of a child. For whatever reason, no one tells the paternity testing company that this is the case and so only one brother ends up getting tested.
The results show that the tested brother's DNA matches the child's and so he is identified as the dad. Except that he isn't. His brother also happens to match the child in this DNA test.
What needed to happen here was for someone to tell the testing company upfront that either brother might be the dad. Then the company could test both of them and do additional tests until one or the other brother was ruled out.
At first this might all seem weird since, unless the brothers are identical twins, they don't have the exact same DNA. You'd think this would mean that a DNA test would never confuse two people. And you'd be right if DNA tests looked at all of someone's DNA. But they don't.
Current DNA tests only look at a small fraction of anyone's DNA. This is enough to tell unrelated people apart but it doesn't always work for related people.
Related people share a lot of the same DNA. This makes them more likely to share the bits of DNA that get tested too. And this makes it more likely that they will be indistinguishable on some genetic tests.
So to get the right answer, you need to pick a good testing company and give them all of the facts.