Question about yDNA result if 3X GGrandfather is same person as 2X GGrandfather

[kimspangrude]

Hello,
I am working on a genealogy research project that involves calculating various family scenarios, and  how different relationships would affect the DNA results of descendants. I'm not a geneticist by training, and so trying to work out how DNA results would be affected is really proving to be challenging for me.  I'm not talking about dominant or recessive genes being passed from generation to generation, resulting in syndromes or other abnormal result; this would be highly unlikely anyway, and isn't the issue at hand. I'm just wondering about the DNA configuration itself.

I recently ran across a scenario that involves a 3X great grandfather who, according to family lore, became the father of his daughter's son through an incestuous relationship with his daughter. I know that the yDNA of all the male offspring of this pairing would be that of the father of the mother and the father/grandfather of the son. But would that affect the percentage of yDNA of the offspring? Because if I am thinking it through correctly, the mother would have half of her father's DNA and half of her mother's DNA.  Her son, who is also HER FATHER's SON, would have half the SAME father's DNA, and an additional percentage of the father's DNA via his daughter- the mother of the child.  Would it mean that a generation would somehow not be reflected in the DNA, since technically the child was also the half brother of the mother, thus being from the same generation as she DNA-wise?  And so, four generations later, would the yDNA only reflect three generations? What kind of DNA testing would be the most logical to choose if one was trying to find out if somewhere up the tree an ancestor was the product of incest?  Apparently a 3X great grandson who is a direct descendant of this pairing has undergone yDNA testing, and although the results aren't in yet, assuming that the family story is true, he will have the yDNA of his 3X great grandfather - his 2X great grandmother's father - but if no other family member has been tested, how would he be able to tell if his 2X great grandfather was the product of mother and grandfather's incestuous relationship? If the family story is NOT true, logically, he would have his 3X great grandmother's partner's yDNA. 
Does anyone have any insight that would help to figure out this question? 

[ChristineM]

No expert but I believe yDNA testing will only be helpful if there is a direct unbroken male line from tester back to the relevant gg or ggg grandfather. It is only passed down the male line. Even then you would need another descendant to have tested and possibly more refining tests to ensure they have a recent connection.  If lore is incorrect he may be lucky that a descendant of the actual father has already tested. May still need more refining.

You may need to read up a bit more on exactly how yDNA can be used in genealogy:
https://isogg.org/wiki/Y_chromosome_DNA_tests
https://dna-explained.com/2017/06/05/working-with-y-dna-your-dads-story/amp/

Apologies if I'm telling you what you already know.

Good luck,
Christine.

[kimspangrude]

Thank you for your reply; it confirms what I already know to be true, and that is a good thing. I guess the first step is to wait and see what the results of the currently tested male descendant are.

[bradburyd]

I may be being stupid, but I thought that the Y-chromosome came solely and totally from a boy's father. The boy's mother has no input to the Y-chromosome, irrespective of who her father was.

[Mart 'n' Al]

This might not help.

https://genius.com/Guy-lombardo-im-my-own-grandpa-lyrics

Martin

[sugarfizzle]

This might not help.

https://genius.com/Guy-lombardo-im-my-own-grandpa-lyrics

Martin

  Seen this before, I always get lost halfway through.

Regards Margaret

[familydar]

Ydna tests the y chromosome, which only males have, and it doesn't get "watered down" the way atdna does through the generations, although it can mutate.  So assuming family legend is right, and no mutation, the son of the daughter will have the same y result as his father, who also happens to be his grandfather.  If another male rather than his grandfather is his biological father, then unless that male shares the same lineage as the grandfather (so mother's brother, grandfather's brother, grandfather's father or paternal uncle) then the son's ydna will NOT match that of any other male descendants of his grandfather.

My father and a paternal male cousin match on the y chromosome but their atdna results are completely different, reflecting their shared grandfather but different mothers and fathers.

Jane :-)

[kimspangrude]

I may be being stupid, but I thought that the Y-chromosome came solely and totally from a boy's father. The boy's mother has no input to the Y-chromosome, irrespective of who her father was.

Yes, I know - but thank you.

[kimspangrude]

Ydna tests the y chromosome, which only males have, and it doesn't get "watered down" the way atdna does through the generations, although it can mutate.

Yes, I understand that. I’m asking if there is any way to tell through DNA how many generations have passed, or if there is another way to tell if an ancestors paternal grandfather was his father. Thank you, I appreciate your help.