Advice Required

[Doublemartini]

I wonder if I might prevail upon you more knowledgeable folk to give me your opinions on the following.

I'm not sure of my father's true ancestry. His "father" (paper record) never married my grandmother and there is a 45 year age gap between them. This meant my father had half-siblings 50 years older than himself. A descendant of one of these is my fathers "half" great niece who is still alive. Is it likely that a Family Finder test taken by her will establish whether I am a blood relative in any way, or that she definitely cannot be? My understanding is that if enough DNA is shared it can but that if none is shared that doesn't prove she's not. Am I right?

[dna]

A half-great-niece should share half of the amount DNA a great-niece shares, and that is lots. See http://www.isogg.org/wiki/Autosomal_DNA_statistics for some details.

W. (Mr.)

[Doublemartini]

Thanks for that. This DNA mularkey is often difficult to get your head around!

[nicolam]

Will the half-great niece be compared with your or your father? I'm not sure what the relationship with you would be - half-cousin once removed or something along those lines? According to 23andMe for 2nd cousins the probability of shared DNA being detected is around 99% and about 90% for 3rd cousins. Which would mean there is a very small chance that you could be related but not share DNA.