Help with theory

[Towdlass]

I have a theory that the father of my paternal grandmother might also have been her grandfather. Is it possible for me to be able to tell if this might be true from my DNA results? Please don't be too technical when trying to explain to me. I have to admit that I am still learning about how to use DNA results. Thanks for any help or advice.

[Mike in Cumbria]

I have a theory that the father of my paternal grandmother might also have been her grandfather. Is it possible for me to be able to tell if this might be true from my DNA results? Please don't be too technical when trying to explain to me. I have to admit that I am still learning about how to use DNA results. Thanks for any help or advice.

No, I don't see how you could determine this based on your DNA results.

[rlw254]

Do you mean that the man alleged to be her grandfather is actually her father? Would the birth mother be the wife of the "grandfather" or a woman from a younger generation? It could be possible to figure out but will require a lot of people testing and some luck. A little more information is needed for me to be clear.

Look at her alleged birth mother. Can you find other people who have had their DNA tested that match to any common ancestors in this line? Can you track down descendants of the alleged birth mother and persuade them to test? If you find matches to this maternal line, what about the alleged grandfather's wife's line? Negative results are not very informative but they can help.

It should also be possible to see the difference in numbers, but this will require some luck and a lot of descendant testing to be sure. Though until I can be clear on the scenario it is hard to say what to look for.

[Mike in Cumbria]

Do you mean that the man alleged to be her grandfather is actually her father? Would the birth mother be the wife of the "grandfather" or a woman from a younger generation? It could be possible to figure out but will require a lot of people testing and some luck. A little more information is needed for me to be clear.

Look at her alleged birth mother. Can you find other people who have had their DNA tested that match to any common ancestors in this line? Can you track down descendants of the alleged birth mother and persuade them to test? If you find matches to this maternal line, what about the alleged grandfather's wife's line? Negative results are not very informative but they can help.

It should also be possible to see the difference in numbers, but this will require some luck and a lot of descendant testing to be sure. Though until I can be clear on the scenario it is hard to say what to look for.

If I read it right, the "also" in the original post meant that there was a suggestion that the great great grandfather had impregnated his own daughter.
If so, I don't see how looking at the poster's DNA could help with the query, especially given that this is now  going back 3 and 4 generations. Identifying other descendants of the great great grandfather (not via the great grandmother) and comparing the amount of shared DNA could give an indication, but not with a high degree of confidence.

[Craclyn]

Yes, it would impact on the amount of DNA shared with other matches who are from the same ancestral lines.

[hurworth]

DNA may help you with this.  Could your father or any of his siblings take a DNA test?

[Guy Etchells]

I have a theory that the father of my paternal grandmother might also have been her grandfather. Is it possible for me to be able to tell if this might be true from my DNA results? Please don't be too technical when trying to explain to me. I have to admit that I am still learning about how to use DNA results. Thanks for any help or advice.

In theory it is possible but in practice very unlikely to show anything significant.

I have been looking at DNA results of 5 daughters of one man where one daughter was claimed (by her mother, who had no reason to mention the subject to me (long story)) to be the daughter of his father (i.e. the grandfather of the other 4).

When their DNA is compared there is so little difference it is all within the margin of error and no conclusions can be made.
One thing is certain the daughter in question cannot have the same father as the other daughters as he was in prison at the time of conception, but even that could be in doubt if the birth was delayed.
Cheers
Guy

[Spidermonkey]

In Guy's scenario though, that is the mother and the paternal grandfather conceiving.  However, if the child was a product of incest between mother and maternal grandfather (i.e. her father), then surely that would skew the DNA results considerably?

https://genetics.thetech.org/ask-a-geneticist/identifying-incest-dna#:~:text=Look%20at%20Mom%20and%20Dad%27s%20DNA&text=We%20know%20that%20on%20average,their%20DNA%20with%20each%20parent.

[Towdlass]

Yes I am suggesting that my grandmother's father might actually have been her mother's own father. I realise that this is not good and I am hoping that it isn't true but his wife would seem to have had dementia. Not sure at what age this started but the daughter (my gran's mum) never left home and stayed with her mum even when dad was away on business. Yet she managed to get pregnant twice. She was 44 when her father died and her mother had died 11 years previously. One week after he died she got married and moved out of the family home.
Now, I realise that I am putting two and two together and making twenty two but I thought it was a possibility and might be something worth checking before trying to work out who else it might have been. I understand that this was something which went on quite a lot back then - well, more than we actually think it did. It sounds as though it might be more difficult to prove than I had thought. My father is no longer with us and every one of his siblings have also passed. So I guess it back to the drawing board.
Thank you everyone for your replies and thoughts on the matter.