I have been researching my family line for decades, and have run into quite a brick wall with my earliest known progenitor, Daniel Cooper Sr. (1695-1795). There is an oft-repeated story about him being born at sea, but there is no evidence this actually happened. In addition, I'm a direct male descendant of Daniel and my Y-DNA doesn't match any Coopers besides known descendants of Daniel's, but my Y-DNA does match many folks in the Coryell family.
So if there was a non-paternity event, the question becomes, when? Daniel had 10 children, including 5 sons, but to the best of my knowledge all of the living male Cooper descendants come from Daniel Sr.'s first son, Daniel Jr. If I could find direct male descendants of one of Daniel Sr.'s other sons, I could do Y-DNA testing and we'd pretty quickly have some answers (or at least more data). But without those direct male descendants, can I do anything with autosomal DNA testing?
I'm thinking the answer is "No", but I would love to be proven wrong. Here's why I'm thinking autosomal DNA won't help. Imagine a family tree where woman B had one child with man A, and one whose father might be A, but might be C:
A---B---C?A? | | D---E F---G | | H---I J---K | | M P
So person E's DNA is 1/2 A and 1/2 B DNA (more or less, I know nothing is exact) and for now assuming person C exists, then person F's DNA is 1/2 C and 1/2 B. Continuing down, H's DNA is 1/4 A, 1/4 B, and 1/2 D, and M's DNA is 1/8 A, 1/8 B, 1/4 D, and 1/2 I. Similarly, P's DNA is 1/8 C, 1/8 B, 1/4 G, and 1/2 K.
But if there is no C - if F's father is A - then P's DNA is 1/8 A, 1/8 B, 1/4 G, and 1/2 K.
That means that at 3 generations down, M and P share 1/4 of their DNA if A if the father of F, but only 1/8 of their DNA if C is in fact the father of F. And in this case, Daniel Cooper Sr. is my great(5)-grandfather, so if I find a non-male descendant of one of his other children, that person and I have either 1/64 or 1/128 matching DNA and that just doesn't seem like enough to show up on an autosomal DNA test.
Or am I thinking about this the wrong way?