What is the probability that my relative and I share enough autosomal DNA for testing to detect?
If your relationship is within recent generations (2nd cousins or more recent relatives), testing is almost sure to detect your relationship. Testing will also detect many 3rd cousins and about half of your 4th cousins. It will only detect a small percentage of 5th and more distant cousins. This is because you share an average of 50% less DNA with an ancestor with each additional generation. Here is a chart that shows how much autosomal DNA you can expect to share with different levels of cousins.